In addition to resetting the wifi passwords and the modem, I suggest a couple of other measures:
1. Enable Two Factor Authentication on your account - anyone that tries to log into your online account will prompt the system to send a unique code to your cell phone that you must enter for the login to be validated, otherwise it's denied. This will eliminate xFi as the source of the breach.
2. You need to change the default password on the Comcast gateway ( http://10.0.0.1 , default username is admin, password is password), otherwise anyone on your network will be able to log into the gateway and change anything on it. This will also prevent anyone within your network from making changes to the wifi settings.