@Leakyfaucet, looks like your in the same boat. I would think that the only way to solve this is to put the CGN3 in Bridge mode and connect a router to it, and let it handle everything, wired and wireless. In that configuration there wouldn't be any difference to the downstream devices as they wouldn't know or care if they were connected to a CGN3 or another router, but you would see better performance when running a 3rd party router.
Edit: Here's a thought for both of you. The Mumble application may be running some type of constant comm check in the background which is dependant on the return timing. Don't make the window and you are dropped off the server. I wonder if there is a setting to adjust that timing window within the application that would keep the application and server satisfied enough to keep running? We're probably talking no more than a few milliseconds difference to keep the application running properly.