How do I enable the 4th ethernet port on XB7 modem?

I have an XB7 and I want to use all 4th ports but for some reason the 4th port isnt working. When I checked the router admin page 10.0.0.1 it showed the 4th port as inactive/disabled. How do I enable that?

its the one with the red stripe - https://i.imgur.com/vow2k3z.jpg