Thanks for the links and info, tfunk. I actually had my T-Mobile SIM on Monday, but had too many errands to run, including a leaky car tire, before I had to go into lockdown mode today. Thanks, Covid!
I put the new SIM in the second slot of my modem yesterday, and it didn't appear to be activated. I finally had time to wait on OTR chat this morning, and got it activated. The agent did say the new SIMs were supposed to be pre-activated before going out, but obviously that didn't happen for us.
Long story short, I wasn't real optimistic at first about the new service, but it does seem to work. I still have my external antennas pointed to the south of me where the closest AT&T tower is, about two miles away, and it still works. As far as I can tell, the nearest T-Mobile tower to my location is about 3.5 miles to my east-northeast.
My limited time testing so far shows down speeds of 5-10Mbps, and up speed is less than 1Mbps. I was getting usually at least 6-15 down, and as high as 20-50 down during low demand hours with AT&T. Up speeds were much better with AT&T, usually at least 5-10, sometimes better than down speeds. I guess I will have to bite the bullet and get back up on the roof and try pointing my antennas a new direction in an attempt to get better signal again. Thanks AT&T!