I'm in Germany at the moment, and I'm using a European SIM card instead of my T-Mobile in order to get stuff done. Germany's 4G/LTE is notoriously spotty, but at least I can get 4G most of the time.
You can buy T-Mobile "data pass", but it's ridiculously expensive; the largest pass you can buy is a 500MB, for $50 (!).
The competition is equally bad, of course. AT&T charges $10 per day (!) for international roaming, meaning you're given the same amount of data as in the US.
[1] https://www.t-mobile.com/travel-abroad-with-simple-global