Overseas Work: How to avoid T-Mobile cancelling data roaming?

As a MSC CIVMAR you are probably assigned abroad for 6 months at a time. I don't know of a good solution. Turning off the phone's cellular radio and using just WiFi seems somewhat self defeating as you're still paying for your cellular service but can't use it.

Have you considered Google Fi? Since you pay for all the data you use, they might be less picky about international roaming. Not sure.

One thing I've done is to game the system by downloading huge test files over cellular before overseas travel. This shifts the statistics in favor of domestic usage. I doubt this would work for 6 months, though. 

In the past I’ve carried prepaid international roaming SIMs but I have not seen any really good offerings recently,

If you frequent the same countries (MSC ships tend to do that) you could get a local prepaid SIM for data and use your T-Mobile SIM just for voice and text. Juggling a bunch of prepaid SIMs can be a PITA, I know.