The first thing is that you need to assure that you have no data vs very slow data. Does your phone have a TCP/IP ping utility? Can you ping 8.8.8.8?
If you have voice and text but no data; first try selecting a different carrier. If that does not help, then check your APN settings. If your APN is set to "fast.t-mobile.com", try setting it to "epc.tmobile.com". Also, if the IP mode is set to IPv6, change it to IPv4.
Then try whatever carriers are available. If none of that works, you can call T-Mobile international support at +1-505-998-3793. It's a free call from your T-Mobile phone.