iPhone keeps asking to be activated... activation screen keeps coming

ok so this iphone 6 will go through normal set up process...

come time to activate it does so successfully..

then i hit enable location - get started - bam were in the home screen...

after about 1 minute it goes back to the activation screen and that same process just keeps repeating..

i tried to restore it via itunes... and a factory restore and nothing....

what part would i need to take a look at and maybe replace?

Thank you but that did not help..... I know what you are talking about but it does not help me....

Anybody on here seasoned enough to answer this??

I ran a check on 3utools and its showing a different model and color than the actual shell which means the board has been swapped before..... im trying to figure out why it wont activate with any sim card inserted..

i even put some from overseas just incase its a over sea phone.....

UPDATE::: 11/18/17

I CHECKED THE ESN AND ITS AT&T.....

I SUBMITTED FOR UNLOCK ON ATT/DEVICEUNLOCK

IM WAITING FOR UNLOCK APPROVAL AND I WILL TRY TO ACTIVATE AGAIN...