Some people have signed up for ATT prepaid on a generic phone and sucessfully moved the SIM over ATT. I recommend purchasing a chip from a virtual network using the ATT network --Straight Talk (Walmart); H20 Mobile, etc. Straight Talk is $45 a month for unlimtied domestic talk, text, and data. It has the advantage of being widely available.

I was in Yellowstone last year. I had coverage on ATT outside the park, at Old Faithful, at the Village, and at Roosevelt. There is also some wifi at Canyon Village. I had some coverage in Grand Teaton. The coverage was perfect in Jackson Hole and Red Lodge. I also have a Verizon MiFi and don't think coverage think their coverage was perfect, but I don't swear to it. I know that when Verizon covers some place and ATT doesn't I put iPhone on Verizon via MiFi. I don't recall doing so, but don't want to swear to it.