Description of Problem:

Hello, I've been trying to port forward for my Minecraft server. I followed the Open Ports for Minecraft Server on Routers guide and got to the very end. I'm a bit confused by "Note: Be sure the WAN IP on the router is a public one, otherwise, you will need to open the corresponding port on the modem in front of the TP-Link router." 

Going into my settings, I see the WAN IP is a private one. What do I need to do to "open the corresponding port on the modem in front of the TP-Link router?"

Extra Information:

The server works on "localhost" when I try to connect on the device it's being hosted from

The server works on other devices connected locally to the same network

The router has been power cycled

I have verified that the port is closed through a port checker and my friend attempting to connect to the server