I can backup general consensus by personal experience. I'm using ATT MVNO and I get service in more places than postpaid TMobile. When we went to Mackinaw City up in MI couple of years ago my ATT MVNO had service while TMobile was spotty. When we went to Smoky Mountains earlier this year, again, TMobile had no service by our AirBnb cabin while my ATT MVNO phone had service, which literally saved us an hour worth of driving back and forth to service area - we had to call our AirBnb host twice, once for better directions to the cabin, and second time to get the right code for the cabin, we were really lucky that I had service and could just call the host on the spot, if we only relied on TMobile we'd have to drive to the nearest town for service first, would have been huge PIA.
Moral of the story is, if you don't need reliable service out in the boonies, TMobile is perfectly fine, but ATT sure does come in handy with their better coverage if you need it.