FWIW, our Canadian provider Bell (never had a problem before when they used Verizon and AT&T) have started using T-Mobile for our USA roaming. We only became aware of this when down in Sarasota Fla with terrible service = non-existent in many place and even on the I75 & 41/Tamiami Trail in many areas on those roads with population closeby from Fort Myers up to Tampa. Very very disillusioned with having T-Mobile as a primary "reliable" connection after our experiences with it this past November and December gone in Sarasota area with a good population!
Upon googling our problems when down there others had written that Bee Ridge Road, University and some others even their AT&T doesn't work but Verizon came in good for all in the area.
Just saying based on our experiences and what reviews were written when we searched to see if it was us or a common problem with T-Mobile. Surprised we had no data to even receive emails or iMessage in such a large populated area as Sarasota.