You have to disable DNS resolver in order to set DNS forwarder.
The instruction Step 6 mistakenly states that you have to disable DNS forwarder, it's wrong.
If you don't disable DNS resolver, you will get an error whenyou try to save DNS forwarder settings.
You can use any public DNS like
Level3 4.2.2.1, 4.2.2.2 or
Google 8.8.8.8, 8.8.4.4 or
OpenDNS 208.67.222.222 . 208.67.220.220, etc.,
or
https://airvpn.org/specs/ like 10.4.0.1, 10.5.0.1 etc, I think. (Maybe not if you are not connecting to AirVPN)
in System menu, General Setup, DNS servers