How to keep NAT type on open and fix "UPnP not successful" error?

I'd like to start this off by saying that the gateway I have is a NOK 5G21 with T-Mobile home internet, meaning I have no way of port forwarding or doing any sort of customization/personalization. My current console is a Series X, but I also had this issue on an Xbox One S. I've been having an issue for maybe a week or two now, where my NAT type has been stuck on moderate or strict, and is accompanied by a message that reads "UPnP not successful," making multiplayer games difficult and sometimes impossible to play. Before this, I was playing with no problem for months, even with T-Mobile as my provider. I've already called T-Mobile tech support, who said that everything looked fine on their end. So, now I'm here. Everything I've tried is:

1. Turning the gateway on and off

2. Rebooting the gateway

3. Unplugging the gateway

4. Hard restarting my console (One S and Series X)

5. Unplugging my console (One S and Series X)

6. Factory resetting my console (One S)

7. Contacting T-Mobile

8. Unplugging my LAN cable from both the console (One S and Series X) and modem and cleaning all of the ports with an air duster

There are probably some other things I've done that are just slipping my mind at the moment, but that's most of it. At the moment, I seem to be able to get an open NAT and fix the UPnP error by testing the NAT type every time I turn on my Series X, but I would really prefer to not have to do that every single time. If anyone has any suggestions, it would be greatly appreciated.

EDIT: I just tried clearing the alternate MAC address, that didn't work either.

:)