I believe that’s the VLAN for the WAN, basically you only need to enable that if your ISP is using VLANs, but you want to enable VLANs on your LAN. I have the Telstra version of that modem (I recognise the Technicolor UI) and it doesn’t allow you to use VLANs like you want to.
You could probably set up some routing stuff on a raspberry pi though, and use a switch.
Or if needed put your modem into bridge mode, and acquire a router that supports VLANs, I don’t know of any cheap consumer ones, but I’m in the process of switching to OPNsense with an old computer. Unrelated, but in my experience technicolor has severe bufferbloat anyway.