Bills for disconnected service

I have cancelled xfinity service last month, and received an automated email notifying me that my service was being cancelled on 08/01/2020. I paid my final bill for July service, sold my personal modem to the next tenant and promptly returned the cable box to your office on 08/03/2020.

However, I found that new bills (for whole months of August and September, and a late payment ) still continually show on my account! I was assured by the agent that there would be no cancellation fee since I am using the package without agreement. In addition, I noticed that my account is still showing a connected modem.

My cancellation confirmation number is 1001239430742099. How can I remove the modem device from my account? Should I just disregard all upcoming bill showing on my account? I would prefer not to leave unpaid statements on my account as I'm sure there will be consequences a few years down the line when I need to use Comcast again. I am waiting for your fast reply confirming the cancellation of the service from 08/01/2020 and cancellation of your bills. Thanks!