T-Mobile has no native service in Alaska. It instead uses domestic roaming using their partner providers GCI and AT&T. When connected to GCI, you'll get 5GB of free roaming data at their "top speeds" and then you'll get 'dumbed' down to 3G speeds after the 5GB of full speed data.
AT&T data is capped at 200MB and is always capped at 256KB data speeds, so you really want to go into settings and manually connect to GCI if your phone connects to AT&T instead. Text messages, data and phone calls are all free while on GCI or AT&T.
Note this is no different than other areas where T-Mobile has no native coverage. For example, in Death Valley National Park in California, T-Mobile has no tower and uses domestic roaming on Commnet Cellular, which is another small cell phone provider.
To change your provider on TMobile with an iPhone, go to settings - cellular - network selection - and Unselect "automatic" if you get dumped onto AT&T. Then, wait about 2 to 3 minutes and you'll get a list of providers available including Verizon, GCI, AT&T, etc. If you try to connect to Verizon you'll just get "no service" -- but choosing "GCI" will get you full speed data.
As someone else said, only disable airplane mode in port. When cruising up and down Alaska and not near a town, Disney will turn on Cellular at Sea which is their own cell tower on the ship. The rates for roaming on that are astronomical. They have to turn Cellular at Sea OFF when 10 miles within a land based cell phone tower legally, though. Best advice is to go to airplane mode and just use Wifi when she ship leaves each port.
All that being said, when 5 cruise ships show up to the same Alaskan port at the same time, ALL the cell providers struggle to provide service and ALL will generally have fairly slow service. This is a bandwidth / backhaul issue as the providers don't have enough data bandwidth to handle 8,000 extra people showing up to the same cell tower at the same moment once or twice a week for just two months out of the year. Instead of spending $$$ to improve coverage, the cell providers just provide the slower service when they get overloaded. And note that "postpaid" customers will always get priority over cheaper "pre-paid" customers of the same service. So someone on the $100 AT&T or Verizon super-duper plan will definitely be browsing faster in congested conditions than someone on a Verizon Prepaid, Visible, or Cricket Wireless plan.
