Plex's WAN Discover-ability feature is a little buggy. After you believe you've completed your Port Forwarding steps and have allowed Plex through your computer's firewall, use a tool like GRC ShieldsUp to test that your port is open: https://www.grc.com/shieldsup . Alternatively, since Plex stands up an HTTP server, you can use your cell phone's mobile data to test with a web browser. Visit https://<your-ip-address-here>:32400/ and you should get a response if all is working. You can get your IP Address by asking Google what your IP address is, or from the GRC site linked above.
If all is well, when you have complaints of a client being unable to reach your server, see if they can access it using the same method I provided above for testing from a cell phone.
I have been having problems with Plex on Cell phone and Smart TV devices lately, which seemed to happen shortly after a recent server upgrade. There was an update a few days ago which seems to have sorted that problem for me, as I haven't needed to restart my Plex server. It seemed to be triggered whenever the Transcoder was triggered.