A few questions:
How long have you owned/used your own modem? Might it still be under warranty?
Is the Comcast modem you are now renting a combined modem & Wi-Fi router? If so, are you aware that Comcast is likely using the Wi-Fi router to create a public Hotspot in your home?
https://www.extremetech.com/computin...nd-of-the-year
https://arstechnica.com/tech-policy/...blic-hotspots/