TM has been doing a LOT of coverage expansion the past year. I would assume sparsely populated areas would be at the bottom of the list. Unfortunately, you'll have to pay the big bucks and switch to Vz for a while. I wouldn't be surprised if even ATT suffers by comparison to Vz. in your new area.
AFAICT, those 700 MHz licenses in Montana have not yet been officially transferred to T-Mobile. When they do, they will be under a very short build-out deadline. The licenses "turn back into a pumpkin" in Jun '19 if the build-out requirements are not met. I would expect to see some sort of T-Mobile signal in Billings by EOY but I may be optimistic. The build-out will probably progress along I-90, 94 & 25 to cover as much ground as quickly as possible. (The Billings license covers part of N. WY.)
For now, the best I can tell, you're limited to the duopoly and Sagebrush Cellular (a.k.a. Nemont). They appear to be a Verizon LTEiRA partner.