Why don't I have 4G on my Galaxy S3?

[AC Question]

No 4G on Galaxy s3

I got an AT&T galaxy s3 from a friend and had AT&T unlock it for me. I put my Tmobile sim card and everything works great except that I can only manage 4G data for about 1 minute, and then it disappears. Did I skip a step or do I have a real issue?

 

[B. Diddy]

Re: No 4G on Galaxy s3

Welcome to Android Central! Are you sure the APN settings are correct for T-Mobile?