Slow 4G connection only, but only sometimes -- SOLVED

Interesting that you see so many different channels. I would guess you are in a higher density population area than where I am. It would probably be beneficial for you to investigate the tower locations about your area. I would recommend going to cellmapper.net and locate the various T-Mobile tower locations around your house. It should help clarify why signal strength is better in one location in the house vs another. Plus if your windows and screens are older make be sure to rule out aluminum screens as an influence on the signal penetration into the home. Given you are a veteran ham radios operator (pretty cool) it would surprise me if you have not considered this already. If you have been through the thread you may have come across the reference I made to waveform.com and their guide for MIMO antennas and the router. https://www.waveform.com/a/b/guides/hotspots

Here the B2 4G LTE band seems the most prevalent and n71 for the 5G NR channel. I have mapped out the tower locations and signals sent out from the 4G and 5G towers here and that has helped me clarify router location in our house. You know how important a good antenna is given your background so looking into the MIMO antennas is a good thing.