You can enable/disable specific LAN ports via command line as follows:

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

You might be trying to resolve an issue with an Ethernet-connected device. If the negotiated speed is causing the problem, you can test forcing 10MB/s Full Duplex connections before setting it back to Auto-negotiation. For example:

robocfg port 2 media 10FD
robocfg port 2 media auto