The resolution to this problem was mentioned on page 1.
You have to request a second modem. The only way you will be able to have your network set up the way you want it and be able to access it from outside is to get two modems. It wouldn't really be secure if you could access your secure network from outside so easily. In order for Comcast to monitor your system and always have access to report a problem they have to ensure the monitoring system is not tampered with.
Call Comcast and request another modem. Tell them you have security cameras and other services that you need to be able to access from outside and because the Xfinity Home Security needs to be untampered with you need an extra modem to take advantage of your broadband service. They are more aware of the situation and shouldn't give you a hard time (for too long).