Cancelled Xfinity Internet and still being billed

I called Xfinity on May 30th 2026 and cancelled my Xfinity Internet account. I spoke to a woman representative who said I could cancel at that time or let the internet account end at the end of its service date, June 9 2026. I said to let the service end on June 9 2026. Because I was using my own router/modem there was no equipment to return so she said there would be nothing else required and gave me a cancellation number. For my Xfinity internet plan, I paid monthly, in advance so I had already paid to the end of the service date, June 9 2026. Later, I received an email bill saying I still had a payment due. After calling Xfinity billing I could not get the person to tell me what the amount was for and why I was being charged after I had cancelled. He didn't seem to care and wanted me to pay regardless even saying they would send the bill to a collection agency. I've been waiting to see if the account bill has been updated to show nothing is owed but the bill amount remains. Something needs to be done about this poor service. I'm still not sure who I can contact to get this resolved.