In order to use Plex via an app on Android/IOS, you have to have the Plex Pass, which is a paid service.
You can buy lifetime (I have it) which pays for itself in about two years, or you can subscribe.
You can access Plex via web browser without having to have a Plex Pass by hitting it at your external IP, but you do need a Plex account if you want to do so in a secure manner.
Otherwise, you can just do a standard port forwarding rule to your internal Plex server's IP and Port (usually 32400).
In Plex Media Server, click the wrench on the top right (Settings). On the left under your server, choose the Remote Access menu item.
On the right side, choose to show Advanced settings.
Confirm your port, as selected.
Then configure a port forward in your router to the Plex server.

[This message has been edited by Synthesis (edited 03-01-2022).]