Coverage question: South Carolina, Moncks Corner/ Charleston region

I'm trying to help out a young friend recently moved to Moncks Corner (zip 29461 ) South Carolina, and working in Charleston. She's been having serious problems with voice/text coverage. (Data hasn't come up, but I imagine it's the same.) Phone is a factory-unlocked Moto X4, missing band 71. The phone/SIM had been working just fine until the move, and I don't have a reason to suspect APN issues, etc.

From looking at maps I can find (T-Mo's coverage map, Mint Mobile's 'native' coverage map, which seems to offer more granular inspection, and the FCC map), it doesn't look like T-Mobile has really built out band 71 much in the region anyway? (so I'm not sure the lack of band 71 is an issue). It looks like around Moncks Corner, T-Mo coverage is just pretty spotty, with some areas still dependent on 1900/2600.

I'm hoping someone might have some experience on coverage in the area, and whether T-Mobile is just a bad option there, or if a newer phone with band 71 is likely to help