Comcast Modem Can Access internet, Comcast-certified modem can not

Can anyone explain why an Xfinity Gateway modem/router works flawlessly to provide stable internet in my house (no events recorded in the event log either), but three different Comcast-certified modems and two Comcast-certified routers do not? Is there some sort of code attached to the Gateway modem/router that allows access? I also noticed that the ip addresses of the outside modems are different (192.168.100.1) vs. Gateway (10.0.0.1). Would that make a difference? 

I have been working on this problem for weeks (going on five tech visits, hours and hours on the phone, and complaints to the executive office), and the only difference between a stable internet connection and constant disconnects is the Gateway vs. non-Gateway devices.