If you own your own router/modem you can use it with whatever ISP you wish. Comcast isn't involved with that process.
The information provided by @Nerdburg is correct. If you still need assistance with the cancellation process, however, I can help with that. Just send me a message by clicking on my name, "ComcastChe" then click send a message. In the body of this message please verify your first and last name, and the name of the account holder if it's different than yours.