Charge off, Equipment "not returned", collections!?

I cancelled my service to one of the locations I was paying for. I did this via the chat in the app. I did not owe a balance at the time of cancellation. I returned the equipment and was told I did not owe anything at that time either. A collections account from comcast showed up on the credit report recently. Approximately 6 months after the equipment was returned. I contacted customer service, again via the app, and was told this was a charge for unreturned equipment. I physically handed this off to an employee an Xfinity store. I was unable to find my receipt so went to visit the store. They provided me a receipt of the returned equipment but stated I owed $417 at the time of return, so $220 was removed for the equipment. I asked what that charge was and they said "prorated services". $197 for a month? They said yes. I never got a bill in the mail, I never got an email, I no longer have access to the old account. This has dropped my credit score by 30 points! For equipment that I returned? Or for a single $197 month? I was told I would get an e-mail following the most recent customer service interaction but didn't. There is no way to get any real help beyond the chat, that has not provided me with the information I need for a resolution.