You can enable/disable specific LAN ports via command line as follow.
To identify the port use the command
robocfg show port
Once the physical port is identified, to disable (replace x with the port number):
robocfg port x state disabled
To enable instead:
robocfg port x state enabled
NOTE: you might be trying to fix an issue with an ethernet connected device. If the issue is the negotiated speed you can also try a dfferent approach like forcing 10MB Full Duplex before setting it back to Auto-negotiation again e.g.
robocfg port 2 media 10FD robocfg port 2 media auto