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.