@ganemesis - It can be very difficult to troubleshoot network issues due to the large number of variables involved. It sounds like there may be some issue bridging between the wireless network and the LAN. Do you know if you setup the AC4000 so that it bridged, or is it using NAT?
It might also help to look at the last suggestions on the WiFi troubleshooting page, under the “Router Issues” and “Sync without Bonjour/Zeroconf/mDNS” sections.