Had been using this set-up with no problems for several months. However, this week we had to replace the Airport Extreme after it died inexplicably (under warranty fortunately.) The replacement though is not talking to the modem. Airport Utility comes up with a message telling me the WAN cable is not connected. Sure enough the LAN light on the modem does not come on at all. I've tried using different cables but there's still no sign of life. The corresponding socket on the router seems to be operational as it lights up briefly when the router is started. So far I'm starting to think there's an issue with the LAN port in the modem.

Anyone got any other ideas I could try to test this out?