Check you firewall logs or analyse the traffic coming to your plex server on port 32400.
And as OttoKerner said, you also need to allow access from your source address, and all other sources you plan on using.
I agree with gby, everyone else seems to have gone off topic a bit.
I've heard Plex runs AWS instances so fairly unlikely you can block to a small enough range to be usable. However if you did like gby said and logged traffic to your plex port (32400?) you could be able to find something usable.
