Tmobile prepaid has had international roaming for at least 2 years. Worked for me in Jamaica in 2012(Saved me there when my international sims did not work). Phone also registered in Nigeria and UK last year. Did not need to use it there since I had sims for those countries. Like you said, the tmobile prepaid roaming rates are steep, but it's good to know one has the option.
It should work in those countries. See below.
http://wac.04c7.edgecastcdn.net/0004...llingRates.pdf
Phones: Xiaomi Mi Mix 2, Samsung Galaxy A50, ASUS zenfone 3,
Sim cards: AT&T (Contract), 3 UK, Piranha Mobile