This is what I have done after a similar problem. I have AT&T Fiber, your modem/gateway may be vastly different. This may give you pointers where to begin to look. In the gateway address (mine is 192.168.1.1), navigate to Firewall (tab across the top), select NAT/Gaming in the second row of tabs; click Manage Custom Services; provide a name you can remember; for Global Port Range, select a 5 digit port and enter into both blocks (only forwarding ONE port); add 32400 as Base Port (assuming you have not changed that in your Plex Setup); Click Add, Click Return to NAT/Gaming. There select the name you were to remember in the Service Box; Click through for your Plex Server Address/MAC Address in the Needed By box; Click Add.
Basically what is happening: The first step(s) tell your modem when it sees traffic on the 5 digit port you specified it 'translates" that to the Base Port (32400 for Plex unless you have changed it). It then sends that traffic on through your LAN to the specified machine you identified in Step 2.
No need to reboot router or machines. If you were successful, you should see a green check mark on the Remote Access line under the Server Settings.
If this is all greek to you, let me know what your router make/model is and I might be able to walk you through (or point you in the right direction to a link, since I would not have the router you have). For some strange reason, I’ve never had to specifically port forward until about 3 months ago. My router and Plex “just figured it out”. Once you get a port forward set up, you should never have the problem again.
