DLink N750 Router Fails To Detect Modem

I've had a D-link N750 DB running smoothly for about a year now. Suddenly the other day, for no apparent reason, it dropped its connection to the modem and refuses to acknowledge it anymore. The router still broadcasts the network ESSID and accepts external connections.

First, the cable modem appears to work just fine, as I'm able to hard wire with a laptop and get consistent internet service. Second, when in the router settings, the ethernet connection status reads not detected, so it seems to me that the router hardware has no idea its got anything plugged into it.

I've tried the standard power cycling methods in various orders about a dozen times. I've replaced cables and done a hard reset of router settings. No dice.