Why can't I connect to the T-Mobile network with an unlocked AT&T i747s3?

[AC Question]

cannot connect to tmobile network with unlocked att i747s3

I cannot connect to tmobile 4g with my unlocked s3. it is originally from att. I love not using my data don't get me wrong but not being able to use my phone for the most part unless I have access to wifi stinks. any help is appreciated