Let me start by saying that any STB should be connected directly to either coxial or the 2Wire (RG). Putting anything in the middle is asking for trouble.
From your post it sounds like your RG is in one physical location and the N900 is a second. Use the Livewire to connect the WD TV Live to the N900. That should get your WD TV Live working correctly.
The general concept to use when building your network is anything that MTS supplied connects directly to the RG or coxial. Anything that you supplied which needs Internet/LAN access connects to the N900. With this seperation it won't matter if the RG or the N900 is handing out IP address (DHCP) since anything that needs to talk internally on your network is under the supervision of the N900 anyway.
One last comment, if you are running a seperate WiFi network on the N900 you may want to turn on the WiFi built into the RG to avoid competiting with yourself for signal strength.