EX8000 Extender internet over Ethernet

I have an EX8000 WiFi extender connected to a D7000 router. Connected to the extender I have a Synology NAS and 2 imacs connected via Ethernet however one of the iMacs has dropped internet connection over Ethernet yet the other iMac and NAS are both still connected?

When I log into the extender I can see the iMac that has lost internet connection doesn't have a virtual MAC address?

Any ideas?