Unable to remove Installment on my line once the device is returned.

I upgraded my line with a new device in Bestbuy on Aug 8th, 2022, and activated it and on Aug 11th, 2022 I returned the device in BestBuy. But on my ATT account still, the installment plan is shown. 

In Sept when I contacted the ATT customer they made and requested and gave credit for the installment and told me the agreement will be removed. After a few days again I contacted ATT customer care and they said we requested and gave the CRM number,  it will be removed by the next billing cycle. In Sept bill also I had seen the installment plan and they charged me. 

This time when I contacted ATT customer care they said they are unable to verify my purchase can you go local ATT store and show them the proof from Bestbuy the invoice so the local ATT manager can reverse the upgrade. 

When I went to the Local ATT store they said we can't reverse the upgrade but I can keep the comments as I have seen the original return receipt with details of the return and told me to contact ATT customer care. 

Again when I contacted ATT customer they said we can't do anything and that I need to contact the Bestbuy team they activated the device. 

But Bestbuy's answer we make a request to ATT that if they approve we can upgrade the device to the customer and Bestbuy doesn't have the right to overwrite the ATT system. As they are a third party, we can't reverse the upgrade as proof we gave the return invoice to you and you can submit it to ATT. 

How should I resolve this issue?