Tmobile Note 4 on AT&T topic

Hey guys, I have a buddy who works for rent a center, he hooked me up with a tmobile note 4 and assured me it would work on AT&T. I've tried everything, it connects just fine, for about 3 minutes and then disconnects? But it is reading my sim. After 3 minutes though it disconnects and reconnects constantly. I'm beginning to think I need to sim unlock it.

Any ideas? Or has anyone gone through this? I'm not getting a sim unlock message either? Even after a reboot. And the phone is stock. Not rooted yet. Will root sim unlock it? With a different rom?

Any help is appreciated.

Thank u

Zound