I still getting bills after the service was canceled

Who do I need to contact to resolve this issue??? Xfinity billed me two extra months after I canceled my service. I called them to cancel the service and get a shipping label to return the equipment. I got the confirmation for the equipment return.

I contacted them to resolve the issue, but they said the agent did not cancel the service. Therefore I am responsible for the extra month that I did not use and don't have the equipment. They will not resolve the issue. Very frustrating!