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