Removing modem rental

Just spent an hour with the chat Assistant trying to remove the modem rental from my bill. I had purchased my own Netgear modem and returned the Comcast Cisco modem two months ago. The Devices section in my online account showed the change but I continue to get billed for rental. The chat Assistant could not seem to understand that I had only one modem, i.e. the one I purchased and insisted I had two modems even though I am using a Negear modem that Xfinity does not supply. He finally told me to go back to the Xfinity store where I had returned the modem to get proof. I said this was unacceptalbe and would escalate. Still trying to reach a live person to fix this.