You have to bring your own modem. Then you bring your own combo unit or bring your own modem that you can plug a router into.
All the modems I see support bridge mode. When enabled the Comcast device doesn’t do any of the routing at all. Your own device gets to do that instead.
Yeah, even at my father's place (I've had different providers than Comcast for personal reasons, but FWIW same story) I've never had a problem just plugging in a router of my choice and using that instead. Makes it a lot easier to handle quirky setups between modem swaps anyway.