nf18acv - LAN VLAN Setting - is this actually useful?

Got a spare one of these knocking around at the moment I I wanted to try and isolate a network for some testing.

My R7000 running Tomato allows for tagged and untagged networks to run on the same ethernet ports.

The nf18acv has a "Lan VLAN Setting" option but there are no details or instructions on what it can actually do. Has anyone had a play with this?

My ideal would be a

tagged VLAN X from R7000 -> NF18ACV port 4.

Normal LAN untagged R7000 -> NF18ACV port 4.

Norma LAN on Port 1 of NF18ACV

Untagged access to VLAN X on ports 2 + 3 of NF18ACV

This would allow me to use the R7000 to provide all DNS and DHCP services regardless of endpoint network. It can also handle any internetwork routing.

I have a feeling my best option is to run the nf18acv as its own network with its own IP address range rather than having it run as a VLAN.

R7000 -> NF18ACV WAN Port

local private LAN X DHCP on NF18ACV

Ports 1-4 on NF18ACV on private LAN X

Damn I wish there was a ddwrt/openwrt build for the NF18ACV

Any thoughts that don't require additional hardware.