iPhone 6s IMEI not recognized for unlocking

Hi I have an at&t iPhone 6s that I have been trying to get unlocked for a month now. I am an at&t customer and I was eligible for an upgrade so I bought an iPhone 8. I am currently using the 8 and have been since September but I have not had any success trying to unlock my old iPhone 6s through the portal and no one has been helpful over the phone, they just keep referring me to the portal page that says it does not seem to be able to locate that IMEI which it should since I was using that phone for nearly 6 months with my at&t contract.The Phone is paid in full, not stolen, I meet all the requirements but still no luck unlocking this phone. I desperately need help with this!!! I know I am using the right IMEI number [edited for privacy – please do not post personal information]. I have tried everything other than walking into an at&t store which will be my last hope!