How to block external internet access for specific devices with Xfinity Gateway XB8

I recently switched from my own cable modem and router and started using an Xfinity XB8 gateway. On my old Asus router I could block my security cameras from reaching the web, essentially limiting them to in home network only. I would like to do this with the Xfinity Gateway but I can't figure out how to do that or if it's even possible. I would think as much as they push these gateways as the best security and they can't do this and my cheap old Asus could, well that isn't good. Any help would be appreciated. Thank you.