Cant connect to Synology RT2600ac

I am unable to connect the 5Ghz WIFI band on my RT2600ac WiFi Router which is Wifi5 compatible. I contacted tech support at Synology re. this issue. he recommended I change the 802.11 mode from "ax" to "ac" for this to work. There does not appear to be a way to change this setting from the T Mobile Internet App. Form the internet, it appears other Wifi 6 compatible TMobile Gateways/Hotspots have the capability to change the "802.11 Mode". Is this possible with the TMO G4AR gateway? 

@shorerider The TM Gateway and Synology wifi router both act as DHCP servers, so they both hand out IP addresses when devices connect to the wifi network. This is problematic and called "double-NAT." If I were you, I would do a few things:

1. make sure the ethernet cable from the gateway is plugged into the blue WAN port on the wifi router
2. put the wifi router into “bridge” mode (usually under network settings
3. restart both and retest

Then, your gateway will be the only one handing out IP addresses. Also, make sure you set up seperate SSIDs on gateway using the TM Internet app.

Let us know how it work. 

I think the tech support guy you spoke with isn't very good.  I say this because "802.11ax" and "802.11ac" are hardware specifications that can't be changed with a setting.  Both support both 2.4 GHz and 5 GHz bands.  

If you were using this router with your previous ISP, and if you can connect the router to the gateway with an Ethernet cable, that should be all you need to do.  

If you are trying to connect the router to the gateway via Wi-Fi, I think you need to configure the router as a Range Extender (but I am not certain.)  It seems to me that if you connect via Wi-Fi to the default T-Mobile home internet network, which is both 2.4 GHz and 5 GHz networks, your router should broadcast both, but I don't know.

I hope another helper will jump in here.  Otherwise, you might need to call Synology again and hope for a different support person.  

EDITED TO ADD:  It took me a long time to compose my reply, and then I saw @copz1998 had already replied.  I’ll leave mine here though, hoping it might be of some value.

Thanks copz1998 and nc1037:  There some more information I need to provide. I am switching over from Startlink due to cost. The Synology router has been configured to run in Repeater mode while I have been on Starlink for 2 years and is configured like that as I switch to the Tmobile 5G GW. It has worked fine on StarLink which is Wifi5 I believe. En Repeater mode, the DHCP server along with a lot of features are disabled, i.e. the parent GW hands out IP addresses. The Wifi diversity and beamforming functionality still operate by default. Since the Tmobile 5G GW is on the second floor and my office space is on the first, there is no wired ethernet and I don't plan on any, so wireless is the only way to go. 

I forgot to mention that I split the Wifi bands on the TMobile 5G GW each with a unique SSID, so I could control the reconfig on the RT2600ac. So after a "reset to factory settings", I am able to configure the TMobile 5G GW 2.4Ghz band successfully, i.e. the RT2600ac searches for potential parents and displays a list from which I can see and select the TMobile GW 2.4GHz SSID. If I stop at this point and do not attempt to config the 5Ghz parent, every thing is copacetic.  If however, I search for 5Ghz parents, the unique SSID I configured he TMobile 5G GW is not found. I even added the SSID as if it was hidden (an option of the RT2600ac) and it says "searching" and the whole RT2600ac becomes unresponsive, requiring a hard reset to factory defaults to start over again. I have been through this process about 8 or 9 times.

@shorerider thanks for teh update. Lets look at the issue from the metrics. Using your TM Internet app, go to the home screen > MORE (bottom) > Advanced Cellular metrics > 5G - what are your reading for RSRQ, RSRP, SINR, Band

If you have a weak 5G signal, that will be an issue. Also, on the TM home screen, what does your Signal quality show?

It is a good idea to split the bands like you did, which helps to diagnose issues. You might want to delete your existing SSID and try just one with the Frequency Band left as "Automatic" and let the gateway steer the 2.4 and 5Ghz devices accordingly. 

After the gateway reset, there should be a default network which uses both 2.4 GHz and 5 GHz. Can you connect the router to that one? 

Here is what you requested. The signal strength screen is very good. RSRQ -10, RSRP -96, SINR -10. I do not believe that a weak 5G signal is the issue; what happens on the cellular side should impact connectivity on the Wifi side of the GW; it may impact performance, but that's not the issue here. In any event an Ookla speedtest on my cellphone which readily connects to the 5Ghz Wifi band reports 248 Mbs. The phone is less than 3 feet away from the RT2600ac Router

Your RSRQ of -10 is great, RSRP of -96 is mediocre, SINR of -10 shows there is some signal interference from other sources that will hinder performance. 

Just to confirm that you can connect to all devices now? Take your phone and walk farthest from the TM Gateway and wifi router and try Oookla again. Unless you have a 3k sq ft house, your download should be similar to >200Mbps

CORRECTION: what happens on the cellular side should NOT impact connectivity on the Wifi side of the GW

I think you've gone off topic. This is a connectivity issue not a performance one. I get decent to very good performance through out the house. I should have mentioned, I moved the 5G TMobile GW into the same room as the Synology router, not even 15 ft away and still the 5Ghz Wifi Band SSID did not show up in the prospective parent GW list