I was having some trouble getting the remote access working for my plex server and realized I had setup the port forwarding incorrectly within my ZyXEL C1000Z modem. I was entering the manual static IP of the server, the starting and ending port of 32,400, the protocol TCP/UDP.
I then left the Remote Port and IP to All Ip Addresses. I applied thinking I was set. Unfortunately, this did not seem to work.
I then enabled the DMZ for my server and tried again and it was fine. I had another router so I set it to bridge mode, disabled the DMZ, reset the manual port forwarding and it still didn’t work.
The fix was making sure to choose Define IP Addresses within step 4. of the Port forwarding page. From there I left the remote Ip address to blank and set the remote port range to be the same as the LAN port range info. That fixed it.