CaptainSTX
Part of the Furniture
Will it work if left in router mode but still not acting as a router?
IE. DHCP server turned off, and not acting as the default gateway?
Edit:
So I tried it but it seems to break more than it fixes.
Guest networks are unable to talk to the router now.
This prevents them from getting an IP address from the DHCP server. This I can probably fix by letting the Asus give out DHCP responses on the subnet but eh . . .
Might just be better to cave in and get a different access point.
Something that works properly with guest networks or assigns VLANs to an SSID.
You are on the right track to set the AP up as a router. Just double NAT it behind your primary router in its own subnet. With this configuration both the primary and guest networks will be blocked from accessing any resources on the primary network. If you want to be able to access any primary network resources from the double NATed router you will need to set up custom routing using iptables.