In general terms, are you looking to set up a DHCP relay via LAN (multiple VLAN’s) ? i.e routers use IP helper addresses to forward broadcasts to another server or router on another network via LAN.
Most (some) people use Raspberry Pie or simular (Routing Switch – DD-WRT ) for this sort of procedure.
A little more detail about what you are trying to achieve may assist guidance given 
—–
Q. Does anyone know if can add a IP helper on rt-ax86U ?
A. No idea but can check with ASUS if there is no responses 
In a completely sane world, madness is the only freedom (J.G.Ballard).
You need to login in order to vote