unlock router

AT&T router was used several years ago with AT&T contract.

The service was stopped a while ago.  I want to unlock the router.

I have tried several times online.

I got several different denial messages. One time it said the device was under contract. NOT!  Another time it said it wasn't an AT&T device.  NOT1 It has AT&T all over it.

I need help.  Very simple  IMEI [Personal IMEI Information Removed]

What is needed to unlock?