I did hack this together about 9 months ago
https://github.com/Fma965/ASUS-DSL-AC68U-VLAN
https://github.com/Fma965/YazDHCP/commit/71151388a8a7a1b1cbe88e538715127c8ca93898
But I haven't updated it since so it might not be relevant for newer versions of Asus SW or anything any more.