This is the T-mobile claim of coverage. It is inadequately covered using roaming on your phone by At&T officially, but you will find that GCI takes up most of the slack where AT&T won't work (almost everywhere metropolitan). If you get 3G in any location you are doing great. Sometimes I get LTE but usually E or 3G and one to three bars of connection in ANCHORAGE (the largest city in Alaska)!
I'm locked in with iPhones that I owe for and five lines (three are family in Oregon where I picked up T-Mobile, (They don't have any T-Mobile stores up in Alaska at all - even though they advertise T-Mobile on the TV all of the time), and two lines here in Alaska for my wife and I. We pay for our phones (about $30 a month), and then the lines and data, so we pay $333 for service monthly (not great for what we get).
Best answer by stevetjr
One would have to think TMO plans on deploying 600Mhz in AK otherwise why buy it. 600 is such a low band that it wouldn't require the tower density had they had to go in there with their standard bands 4 or 2. I suspect one of the other challenges with data especially is not necessarily the tower but the backhaul from the tower especially anywhere outside of Anchorage or Fairbanks. I suspect this because GCI probably has a very limited backhaul and probably why they don't allow data roaming even 2G/3G because it just can't handle the load and would impact their own customers.