HOW CAN I UNLOCK A IPHONE with a carrier att lock

Yea honestly you will have to work with the carrier on that cause apple doesn't have any control on carrier lock. Apple can help with activation lock but not carrier lock.

Here is the relevant resource which is also above;

How to unlock your iPhone for use with a different carrier

https://support.apple.com/en-us/HT201328

The support article you came here from should provide you with the steps needed to try to resolve this issue. Have you tried them all yet? If so, and they didn't help, I suggest that you contact Apple Support directly. To do so, simply select the "Get Support" link in the upper right-hand side of this forum's web page

If you take it to your carriers local store they will sort it for you, the contract will have to be over though.