Returned Equipment

I cancelled my service with AT&T on 8/20/2020. I was told by AT&T to return my equipment to a UPS or FedEx store. I returned my equipment several days later to a FedEx store. On 10/19/2020 AT&T charged me for the equipment saying I never returned it. I called AT&T telling them I have the receipts from FedEx and they said they need the tracking number. I told them my receipts didn't have a tracking number and they told me they could not help me. I read several posts and found out that they (AT&T) does that so they can charge customers for the equipment. Does anyone have any advice on how to solve this?