@steakster is right.
I misread and answered your question thinking you had an issue with your home router. For a cable modem, it has to be compatible with Comcast and that its mac address must be registered with them.
I had done the same thing with an Arris modem I had, before I switched over to business service with Comcast. It is more expensive but business service gets higher priority and better customer service over normal residential service.