Really depends on how your router handles UPnP requests. Some are pretty flaky which is why we generally recommend using manual port forwarding. That check box tells the server the router has manual port forwarding set up and it will not request a port be open via UPnP
Nowadays routers will disable automatically created port forwardings after a while.
The recommendation is to create the necessary portforwarding in the settings of your router manually, and then inform Plex about the parameters.