Having setup my M9s in router mode, using the ethernet backhaul. 
I have forwarded the VLAN correctly and both the guest and the private network are working fine.

Now I want to switch to a on network DNS server (namely a Pi-Hole) and every thing collapeses

The pihole is in my private network, which makes it unreachable for the guest network.

If I also add it to the guest networks VLAN it will get the same IP on both and it becomes unreachable.

Also I cannot give the guest network a different set of DNS servers to connect to.

Please advise on how I can:

* Make a device accessable from both private and public networks

and/or

* Have the private and public networks handout different IP ranges

and/ot

* Have the private and public networks handout different DNS servers via DHCP

Kind regards