A simple Google search has revealed a process whereby this can be done (weirdly, it isn't something that can be done via the HTTP GUI), but there is a strange command in the instructions:
3) Paste in the following lines changing primdns and secdns to whatever DNS servers you wish to use (use the little icon at the top-left of the command prompt window in order to paste). You do not need to edit the line starting with "dns client dnsadd". The example below sets your DNS to the Google Public DNS service. Use "none" for secdns if you don't have a secondary DNS server.
dns server config state=disabled dhcp server config state=disabled dhcp server lease flush dhcp server pool config name=LAN_private primdns=220.127.116.11 secdns=18.104.22.168 dhcp server config state=enabled dns client flush dns client dnsadd addr=22.214.171.124 <-- What is this for? saveall exit
I have indicated the line above, and cannot figure out for the life of me what this is for?
Am I missing something blatantly obvious?