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 it (replace x with relevant port number on your router):
robocfg port x state disabled
To enable it:
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/s Full Duplex before setting it back to Auto-negotiation again. For example:
robocfg port 2 media 10FD robocfg port 2 media auto