What am I being charged for?

We cancelled U-verse June 2nd 2020, and returned the device.   Each month since then ATT charges $72.74.  We have spent several hours on the phone with ATT agents to request the charge be stopped and ATT credit our payment card for the monthly charges which now totals $509.18   Usually the agent transfers the call to another agent who eventually hangs up.  Any one out there that could give advice on how to solve this issue?    Thanks.