Verify that your Server is listed on your Devices page. Disable any VPN on your computer or router. Disable any proxies being used on your computer or router. Make sure you aren't requiring secure connections on the server while using an app that doesn't support them (see below)

How do I connect to Plex server?

In your Plex Web App, go to Settings > Server > Network . Enter the IP address from step one into the List of IP addresses and networks that are allowed without auth setting and save.

Why does my Plex server Unavailable?

Make sure there isn't any VPN software-enabled within your device and make sure to disable any proxies. Also Ensure that you are running the most current version of the Plex media server, so the data within your Plex isn't outdated.