I have Tmobile and it is sketchy.

In Peru, it offered 4g services(no LTE)and used it alot. infact I uploaded over 100pics to FB on the International Data. No additional charge

But In the Phillipines, it didnt work. Free SMS texting worked, but NOT Roaming Data
In Thailand, it didnt work, Free text, but no data
In Singapore, it did work 4g, no LTE
In Japan, it didnt work
In the Dominican Republic, it didnt work, only text

I think on certain networks, you need to add or change the APN. But unfortunately, I do not know how.