You probably need the APN settings updated for mobile data. If you contact them they can push the settings to your phone, alternatively you can do it manually by going to go to Settings > Wireless Connections > More > Mobile Networks > Access Point Names
Name: T-MOBILE
APN: fast.tmobile.com
Proxy: <Leave in Blank>
Port: <Leave in Blank>
Username: <Leave in Blank>
Password: <Leave in Blank>
Server: <Leave in Blank>
MMSC:
http/mms.msg.eng.t-mobile.com/mms/wapenc
MMS proxy: <Leave in Blank>
MMS port: <Leave in Blank>
MCC: 310
MNC: 260
Authentication type: <Leave in Blank>
APN type: default,supl,mms
APN Protocol: Leave it to the Default one