It slows down to 3G I believe. I maxed out my 5GB in Anchorage on that trip and was still using 3G. I just got back from 10 days in Fairbanks and maxed out my 5GB again, and it went back to 3G. If you're at a hotel, use your wifi and you'll get through just fine.
I'm on a Galaxy S9+ which isn't 5G capable, but speeds are good, the same as if I were home in Hawaii. I wasn't connecting to the Hotel WiFi because I have unlimited data. I got the alert on roaming and was cast back into the early 2000s. If you end up visiting here, make sure to take advantage of WiFi where available.
I'm visiting Anchorage and just got a text saying I have used 4GB of my 5GB roaming data. I am connected through GCI, but it seems it is still considered roaming to the TMobile systems. The partnership must be limited to being able to connect through and roam on GCI, not to extend TMobile service. Their service map is clearly misleading in that you can connect, but it is not TMobile, and you're capped at 5GB.
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