Do you need technical support to change a gateway to bridge mode?

Hello,

If I change my gateway from routed mode to bridge mode, and disable DHCP, should it function properly when I connect a router (WAN port) to the gateway's #1 LAN port (same subnet)? Or, will I need to contact Comcast technical support so they can change what ever they need to change on their end?

After I've made the changes, not only does it not work, but I can't connect to the gateway's UI with a client set for static IP.