5G home internet keeps dropping

I suggest a couple of actions to see if you can get a better idea of what is going on. Open a command window and run continuous pings to say the quad9 DNS server 9.9.9.9 or Cloudflare at 1.1.1.1 and let the pings run. When you halt the pings you can see the success/failure rate for the given period of time, average latency etc.. When you go into the HTML interface of the T-Mobile router look beyond the overview and review the current cellular statistics. Use the refresh switch or refresh the page itself or use the F5 option key to refresh the statistics. Do so several times to confirm if the send and receive counters both increment. It would be interesting to know if the  router is  sending but nothing is  received or vice versa. Since you have your wireless device connected, via ethernet to the router be sure to look at the RX/TX counters for the ethernet connection between the two on both sides of the link. Do both agree on RX/TX values and if there are errors recorded or not. If you have multiple clients run pings back and forth from one to another on the local LAN and have each also ping the router gateway IP address. Evaluate the viability of the local and remote traffic and see if there is possibly some communication issue between your local wireless solution and the T-Mobile router. It might be interesting to see if there are search results for others that use the same wireless devices and the T-Mobile router. The T-Mobile router is 802.11ax capable so using a client direct to the router WLAN with a separate SSID might be good to also test with parallel to your current solution. If a wireless client connection direct to the T-Mobile router continues to work while a wireless client upon your personal wireless device does not that would be a good data point. There might be some operational compatibility problem between your wireless device and the T-Mobile gateway. It might be as simple as a software upgrade of the personal wireless device or the T-Mobile router. I can only speculate but if you get more data points it should help clarify where the failure takes place. Record your data periodically and see if things change over time. Taking a screen clip of the overview, status and statistics reporting at each interval and making a document for review will help profile the operation to know what is taking place over time. The more you keep an eye on it the more you will "know". It is not what we think that matters, it is what we know that will make a difference. I also suggest to record the statistics recorded before you reboot the T-Mobile router. FYI, from time to time, ok more frequently than I like, I have seen the T-Mobile router HTML server refuse show squat. It will render the page contents at times but NOT any vital statistics. It can report no connection, no statics counters etc… I believe if you allow the webpage to stand idle for a bit then the data goes stale and you get squat. In that case just close the tab and open a new browser tab/window to the 192.168.12.1 address again. You can always tell when it is in that state by opening a terminal window and sending pings to one of the DNS servers. If your pings are successful but the HTML page rendering reports not connected well the basic cached information seems to render but nothing else so it can be deceiving. ¡Buena suerte!