Im also having a problems with Airplay and network connectivity with my Marantz SR5007 receiver, which is hardwired connected to my Airport Extreme. The AE provides the wireless internet and has other wireless devices connected to it (Apple Server, printer, etc) I have a Comcast / Xfinity gateway modem / router (Its an SMC model D3G, not an Arris). When first installed (by a professional) the Marantz worked great, perfect network connection and Airplay worked like a charm, never even dropping out. After exactly two months, no network connection and Airplay stopped working. The Marantz was not even pulling a correct IP address.
Its under warranty, so I took it in for service and they say its a problem on my end. (They blamed it on Apple, which doesnt make sense, of course). I spoke to Comcast Signature, we tried it plugged into the AE and directly to the modem/router. both in and out of bridge mode to no avail. However, in bridge mode it was able to pull what seemed like a good IP address, but still no network connection on the Marantz. My installer says they cant get involved with Comcast or my modem. And finally, Marantz says there are known issues with Comcast Arris modem/routers (would have been nice to know that ahead of time), while offering no other help.
My next step is to try a regular modem. Im out of ideas otherwise.