Disable DHCP on XB-8T. How can I do this for starters

I have found I just have to many devices for the XB-8T gateway I have. The gateway is slow, and it only allows so many to be active at once.

In fact I'm wanting to setup VLANs and allow the routers I have manage their own DHCP/subnet.

Has anyone got this type of setup configured. I was told by Xfinity that if I switched to Bridge mode my bandwidth speed would drop considerably.