If you have “force DNS” in the AR300M, then as far as I know your ISP router’s DNS are ignored.
Anyhow, the answer is to login to your ISP router and change the DNS setting there (from auto > manual).
I do not recommend using Google DNS - they are very untrustworthy - try this instead:
as stated, the clients connected via the ar300m use the DNSs as configured on the unit
to clarify, the ar300m is connected to a public, open, un-encrypted wifi network and I have no access to the ISPs local or remote equipment. As a consequence of this I am also unable to raise the issue of the ISPs DNS not being able to resolve some addresses.
I have discovered since my original post (using nslookup), the google DNSs are used by default by the ar300m, it is this default I need to change as it is unaffected by the DNS settings in the GUI.
To add to the original description, I discovered this problem when trying to set up a VPN using OpenVPN from the routers GUI, the address for the London server was not resolved. Using a client connected to the ar300m the address was resolved, as the client was forced by the ar300m to use the google DNSs. I connected to the root account using SSH and used the process resolveip to confirm that the ar300m was unable to resolve the name into an address, I then removed the forced DNS at the ar300m and confirmed that the client was then unable to resolve the address as it was now using the ISPs DNS, so concluded that the ar300m DNS force only applies to the clients connected to it not to the router itself.
I was fooled by both the ISP’s and Google’s DNSs both not resolving the name and as a consequence assumed the value set in /etc/resolv.conf was being ignored. Changed /etc/resolv.conf to point at OpenDNS Home (18.104.22.168) and the issue was resolved.
I also started with the wrong command “resolveip” doesn’t provide DNS information and so I was spending time inferring which DNS value was being used, the command “nslookup” provides better information including the DNS server.