Device unlock not recognizing IMEI code on iPhone 6

I just upgraded my iPhone 6 to an iPhone X and now want to unlock the iPhone 6 so I can do what I want with it.

It was purchased through AT&T and is completely paid off.

When I go through the unlock steps the system says that the IMEI is not recognized as an AT&T device, which is simply not true.

I have been using that phone for almost 3 years on AT&T and now it's being held hostage by AT&T for some unknown reason (it appears that this is a common problem with AT&T customers).

I was going to try and send a PM to the ATTMobilitycare account but they no longer accept private messages.

Can someone please help?

Thanks!