T-Mobile Home Internet Frequently Stops Working

So correct me if I am wrong but it appears you are running your testing with your iPhone 12 Pro Max. Are you sure your speed test is across the wireless LAN out the gateway or is it across the phone's cellular signal? If you want to have a solid test out the home internet gateway be sure that is the path the traffic is taking. If you have a MacBook run the Speedtest.net application from a wireless client or run the speedtest.net "test"  via a web browser on the client. If you launch speedtest.net in a browser you can enable the advanced reporting. 

If you want to get more predictable results it does help to use the same server. With speedtest.net you can use the "change server" option and then select the 3 vertical dots to get the options and select "Favorite this server". Sure you can just use whatever but results may not be very predictable. When you run a test dont run just one run several tests a few minutes apart to get an average. The results are not just about the speed. Look at the Ping millisecond response, the amount of Jitter and loss. Watch download latency and upload latency with different servers. There are several metrics to consider with the speed testing.