Change DNS Address – How to Use Alternatives to 8.8.8.8

Not sure if you'll find a solution to do that on your consumer grade router. Essentially you need a server to sit in the middle of your connection, or proxy to sit in the middle and intercept all traffic going to the Google DNS servers and rebuild the packets to redirect to an alternate destination.

I'm sure you looked into the windows HOSTS file, and realized that it will only supersede DNS requests, which is not going to help you here.

Looks like you'll need to sit a Linux or Server 2003+ machine between your server and the router. Take a look at IPv4 to IPv4 Interface PortProxying.

http://technet.microsoft.com/en-us/library/cc731068%28v=ws.10%29.aspx

The Tecnicolor modem/routers allow you to give one device (by its MAC address) on your internal LAN the "public" IP. This, essentially, puts that device in the DMZ but, at the same time, the device is contactable through your public IP - basically turning the Tecnicolor into just a modem.

As your DMZ device is also a router providing NAT and DHCP I would recommend you disable the DHCP server capabilities of your Tecnicolor router as this will cause problems. I would also recommend turning off the LAN and WiFi capabilities of the Tecnicolor as accidentally connecting to these will only give you Internet and no ability to connect to the other LAN.

Apart from managing the Tecnicolor modem through its GUI you'll never need to use its addresses as your other router is doing all the work.