Ethernet connected device showing as "Offline"

We have a Technicolor BWG router that is working fine with 6-10 devices connecting via WiFi and 2 devices connected via Ethernet.  A third device had been connected perfectly fine via Ethernet but after it was disconnected and reconnected it now appears on the “Offline Devices.”  The device’s connection to the Ethernet port is recognized, though, as it appears as “Active” under Gateway > Hardware > LAN Ethernet.  How do I get the device to be recognized/accessible again?