Turn off the DNS proxy, but there is another issue that's not easy to pick up. If the DNS handed to the router on WAN is the same as the DNS in the DHCP config, when the DNS Proxy is turned off devices will not be able to ping or or lookup the any of those specified DNS servers. To fix you need to add DNS manually on the WAN (any DNS you will never use) and then add your preferred DNS manually assigned on the DHCP server.
I'm still having issues with the DNS Proxy. See this http://community.cambiumnetworks.com/t5/cnPilot-R-Series-Home-Small/cnPilot-r201P-terrible-throughput-when-connected-via-wifi-hard/m-p/117095/highlight/true#M2103
One other thing to note with this issue is that (before the changes above) on cable it doesn't work, but on WiFi it does (from what I've seen).