Semi-Complicated Network

Going down the network wormhole. The Nokia router of course will deliver the IP addresses via DHCP to the local clients on the WIFI & wired connections. It will also NAT the traffic to the internet via the WAN port. With the Asus RT-AC5300 router it should natively behave the same between its WAN port and the local WIFI and ethernet ports. It is not a good idea to double NAT. Under some conditions, UPnP, it can cause problems. From your descriptions it sounds to me as if you do have a double NAT going on. If you enable Access Point mode on the ASUS that might put the WAN port into bridge mode and prevent the double NAT. A chat with Asus support would confirm that. I don't know how you have your ASUS router configured or connected for sure so it is hard to be specific. The router does operate with different modes; Punto de acceso, AiMeshnode, Mediabridge, Repeater, or Router. You have lots of bells and whistles to play with on the AC5300 so it may take a bit of investigation to know how it is operating. You need to be sure of how the AC5300 is configured to be clearer of what can be done. I have seen other community threads about various problems with "routers" connected to the T-Mobile/Nokia router so searching other posts by users with Asus AC5300s could prove helpful. I don't have an AC5300 to test with so I cannot say how well it will play with the T-Mobile/Nokia router.  Again, more research on forums for what others have encountered. The web-gui configuration interface of the Nokia does not show any WDS configuration so using WDS is not on the table. My take is that all the effort and testing might be an exercise in futility and actually not provide you with significant gain. 

Look at the configuration of the WAN port on the AC5300 and how it is configured. You might discuss the options with ASUS support to see what they recommend. Another way to look at using the ASUS with the Nokia would be to connect the AC5300 to the Nokia with the ethernet cable connected to one of the ethernet ports not the WAN port. This would put all the wired and wireless clients on the same broadcast domain. You could then disable the WAN port and also disable the DHCP server so the solo DHCP server on the segment would be the one from the Nokia router. You do not want two DHCP servers on the same broadcast domain. You could then still setup the 2.4GHz and 5GHz radios on the ASUS AC5300 for the SSID and WPA key(s) desired. Keep in mind the Nokia does NOT allow you to disable the wireless radios but you can reduce the power down to 12%. Playing with the power of the WIFI channels could provide a way to help prevent channel overlap. The radio operation at 12% still leaves some signal traffic. If you take care to avoid overlap the wireless channels then you might be able to use the AC5300 mainly for one area and the Nokia wireless bands for another area of the home. It is possible but more work and may still provide little gain. I considered buying a more capable AX Mesh solution for the house to get better coverage than possible with the Nokia router but I have found it unnecessarily as the Nokia is 802.11AX capable and seems to do a pretty fair job providing wireless delivery across our 3300 sq ft home both upstairs and downstairs. I don't have any AX capable clients currently so it would be overkill and only establishing a future proofing solution. I considered an external YAGI antenna for the cell communication as I have line of sight to the tower and that would do much more for me than the challenges of an additional distributed wireless system in the house. The Nokia does not provide you with any advanced configuration options to speak of. I think it is about ISP control of traffic. If you want a more robust internet solution and are willing to pay for the solution that provides such options then that is another option.

Determine if you have a double NAT going on. Then confirm the Access point mode of operation does indeed prevent the double NAT by having a conversation with Asus support. Be sure to only have one DHCP server, the one on the Nokia router. Avoid wireless channel overlap. If you have an Android phone the Network Analyzer application is a great tool to gain visibility of the radio signals and also scan the LAN. Of course this is going down the network wormhole. 

Sorry for the prior long response. With respect to the 5 port switch on Lan2 and the slow down you state. Get into the web-gui of the Nokia and check the stats on the LAN ports. If there are errors or discards that is good to identify. If the switch is managed then look at the statistics in its interface. Don't overlook the statistics of the cellular interface either. The assumption may be that the problem is local or with the Nokia router itself but well maybe not. Check the 4GLTE and 5G signals on the "Overview" tab and make note of the RSRP and SNR values. Compare when the speeds are good vs when they are poor. Also look at the Status tab and compare the primary and secondary signal "Band" for each and confirm also that the PCI for each is the same. Don't assume it is a local issue only. If T-Mobile is working on the equipment on the tower or connection loads on the tower increase that can also impact the performance and speed.

I got onto the T-Mobile home internet router solution in the BETA phase early in 2021 so my hardware is maybe an earlier revision of the router. I am on software version 1.2103.00.0338 currently. I found playing with the location of the router is really important. If you use a laptop with a hardware to a LAN port and keep the router plugged into a long extension cord you can move it around and see the strength of the signals in the web-gui much better than via the generic bars on the top of the router. Take the time to tune it into the 5G cellular signal. I know it will be the secondary signal but try to get the maximum power out of the 5G signal. Rotate the router clockwise and counter clockwise in 5 to 10 degree increments to improve the signal wash over the 5G antennas. I improved the 5G signal ay -5 DBM just by rotating the can and moving to get the best cellular signal. Don't assume anything with that baby. The wireless and cellular antennas are alternating about the radius of the can just under the shell. You want to get good SNR signal to noise ratio. More positive is better with SNR. The lower negative RSRP value is better. i.e. -70 dB vs -100 dB. If you get the router in a location where the SNR improves and can also get improved RSRP values then you will have a cleaner transmission and better performance. 

I made a crude diagram of the layout of the Nokia router. It will help understand the layout of the antennas and help you get the orientation so it gets a better signal strength on the 5G signal. See the attached PDF. 

Now I got what you are saying. When you do the automated setup it may just have you in as admin in effect. If you want to make changes after that sure you need to get the password off the bottom of the can for admin.

Well, I can understand that frustration. At first when the monkey crew was still getting the tower equipment fine tuned it was a bit of a challenge. If you can't get into it to make changes you can reset it to factory default and use the credentials on the bottom to login as admin. Setting up the SSID and WPA key for each channel in the web-gui is not too bad. You should have the 2.4GHz channel and the two 5GHz channels set. I used the same on all three. If you have any 802.11n clients that will need both a 2.4GHz and 5GHz channel the same to function due to how the 802.11n standard works. Newer clients with 802.11ac may not have that issue. Standards for wireless are rather confusing. Loads of fun for tech heads but not for the average Joe.

Wow, thank you for the lengthy (and detailed) response. Your theory mirrors mine in the fact that it seems like the typical double NAT issue I keep reading about in this situation. There isn't a lot out there with this Nokia gateway and our Asus 5300. However, I have seen others recommend putting it in Access Point mode. It sure would be nice if they gave us more flexibility with this stupid gateway. This is why I hate "all in one" products. I don't need their router, I just need access to the internet. Sadly, we do not have many options for internet in this Cox/AT&T monopoly area. More research and trial-n-error on the way I think.

Yeah, I wasn't able to access the web UI for the Nokia gateway because it asked for a username and I never had one when I set it up. When I setup the gateway, it only asked me to create a password. This is why I hate networking, too many layers of annoyances such as that. To be honest, I'm not 100% sold that this will work out for our purposes since we both work from home, stream TV content and I play online games. We also have security cameras, smart home devises and a NAS. With a cable modem, the AC5300 and a 5-port (unmanaged) switch, everything is pretty much flawless. To be fair, I was already prepared for this nightmare before I pulled the trigger. I had already read about how this Nokia gateway is so dumbed down that its a nightmare for power users. I'll try a few things, but I'm fully prepared to pack it back in the box and return it to T-Mobile. TBH its not even the speed or the low cost that is the most appealing. It was the fact that there is no data cap unlike our stupid Cox cable!

Look at the bottom of the router for the default admin password. It is the MAC address of the router.

By the way, I could swear I read somewhere that you CAN turn off the wifi options on the gateway using the web UI. As for location, in its current location, at best it gets about 500Mb down and 35Mb up. Unfortunately our house is old and our entire network is in a closet. So having the gateway in a different part of the house would be a pain in the butt to re-wire. Right now my goal is stability!

I'm almost tempted to not use our router and try to use just the Nokia gateway and the switch… however I know for a fact that the Nokia has a way weaker signal than the Asus. For example, when I first setup the gateway, I played with it using the wifi and my iPhone. I usually get full wifi bars and fast speeds in the back yard with the Asus. With the Nokia, I got one bar and a 2Mb download. LOL