Yellowstone, northern states cellular coverage?

Why ATT? Although service is sparse is some areas ATT treats their customers much better than Verizon. ATT rolls over unused time on their Sierra module. Verizon doesn't. Plus in NW WY ATT service is pretty good. In SW WY ATT seems almost non existant but I don't live there. Since we travel we have both a ATT Sierra module and a Verizon Jetpack module.