First, T-Mobile doesn't have towers in Alaska, so ALL usage there is considered "domestic roaming". There is a HARD 5GB per month cap on domestic roaming--mobile data apparently gets totally cut off once you hit that cap. There is absolutely NO WAY to purchase additional domestic roaming data. Your only option is to buy an eSIM from another carrier. It seems crazy that the service in Alaska (in the US) is so much worse than their intranational roaming (15-30GB, slows after that, you CAN buy high speed passes).
The issue with international roaming is that they get on you (potentially cancelling your account and forcing you to lose your phone number) if most of your usage in 2-3 consecutive months is international. Buying data passes doesn't seem to help.
If you're on an extended trip/vacation, you can easily get caught unexpectedly.
While it's understandable that the don't want people using massive data internationally, they enforced it strictly based on "over half your usage".
So, if you turn off your T-Mobile SIM and have a Starlink and a local eSIM that you use instead for nearly all of your data and calls--but once a month for 1 minute, you enable your T-Mobile SIM to get, say, a bank's verification code, you violate that provision because 100% of your data, SMS, and phone use was international for 3 consecutive months.
It doesn't matter that it was an Infinitesimally small amount, it was 100% of your usage and that's considered abusive.
So , if you're on an extended vacation/trip, you need to leave your T-Mobile SIM turned totally off the entire time -- or, buy a round trip plane ticket to the continental US so you can quickly make a bunch of calls, texts, and download a bunch of massive files to ensure "most" of your usage is in the US.
It's frustrating that they don't appear to have any provision for trivial usage. And, apparently the only way to get SMS/RCS messages is to enable your SIM (most places allow a Google Voice number for two factor or allow the Authenticator app, but a few places only accept a real cell number).
On a two month trip to Canada and Alaska, I ran into all of this.
Despite buying a Holafly eSIM and a Starlink. 🤬