coverage while traveling thru Indian territory

Keep in mind that "Indian Reservations" are not part of the United States. They often have their own laws and their own police force. Do not mess with the tribes of the Northern Plains like the Crow, Blackfeet, and Lakota.

If you want to know about coverage by some private company you are going to have to ask them. If you visit a Reservation and you are disrepectful and start with the stereotypical **** like on this thread things can go downhill fast.