T-Mobile Home Internet Frequently Stops Working

The first test you posted suggest to me the 5G signal is probably the n71 band based upon the results and the number of bars on the phone cellular signal info. The phone may not have been using 5G at the time but for those results the gateway would have had to of been picking up a 5G signal on the secondary channel. If the gateway can produce those results the tower with the 5G signal is probably only 2-3 miles away at most. I suppose if the 5G band is the n41 and you are out on the outer limits of its delivery that could be the case and that might explain things. If the 5G secondary signal comes and goes the extreme speed change when testing would make sense.