ATT Equipment return SCAM?

So ATT told me august 2020 if I return my modem and buy my own id save $10 a month in equipment fees. So I buy my own, they mail me a return box, and I return their equipment. This was all triggered because my equipment broke august 2020 and We went through my options. I said ill buy my own after Hearing I could save $10 a month, but they shipped me another one anyways. I returned that one also. So all I have had since Sept. 2020 is my own router I own and bought cash. It comes to june 2021 and I cancel my service when I move. They start emailing me hey you havent returned the equipment, youre going to get charged. I call them and tell them youre lying I returned it and they tell me oh sorry nothing we can do. This is criminal! I dont have a receipt from 10+ months ago. I returned it to the shipment center to an older guy at a table during peak of covid. And now they want more money from me. Its all so bad! I do not recommend them to anyone after this experience.