Sorry if this is annoying as its an old thread, but I was able to change the DNS servers.
First I flashed the firmware to non MWeb firmware (I used
this guide and it worked down to the letter, except for some post firmware install configuration that is resolved in the comments section), SSH into it, type
menu, then dns->server -> forward -> dnsset (you can then go
list to see the currently used DNS servers) -> flush.
This will remove the ISP DNS servers. Then go to
add.
use set=0, metric=0, intf=Internet, dns=UnoTelly DNS server IP. Leave everything else blank.
Do this for both IP addresses (primary and secondary).
Then keep selecting
Cancel until you are out of the menu and type
saveall.
You don't need a restart. I havent checked if the ISP DNSs will be re-added automatically if you restart the router.
Note that flashing non MWeb firmware will remove FON access.
My chromecast still isn't working with Netflix ATM, not sure why. My phone and PC both work without any local DNS configuration, so perhaps the chromecast is using Google's own DNS servers. I think a VPN used on a router may be the solution, but I don't think this router supports that.