I have installed a simple recursive DNS using unbound to handle my own lookups. It runs fine, and appeared to work correctly with a non-VPN build of tomatousb.
However, I have tried installing a couple of VPN builds (currently Shibby's 1.28.0000 MIPSR2-108 K26 USB Mega-VPN) and have problems.
If the router has DNS set to 220.127.116.11 (Google) then things work ok. But if the router has DNS set to the dedicated internal server e.g. 10.2.4.6 then lookups on the router do not work. Running in the router's tools the command nslookup ibm.com 10.2.4.6 does not work. However, on the DNS itself, running the exact same command works correctly.
What could be causing this?