Typically this is caused by the system not correctly detecting the Ethernet device within the dock. In most cases, completely resetting the dock and ‘cold booting’ the system by following this procedure can help:
- Disconnect the dock’s Thunderbolt 3 cable from both the dock and the system, and disconnect the dock’s power cable so it turns off completely.
- Shutdown the Mac system completely (do NOT just restart).
- Power on the Mac system and login.
- Reconnect the power cable to the dock, and then reconnect the Thunderbolt 3 cable to both the dock and the system.
- Test the previously non-working Ethernet port.