Why it should be this complicated to access your own server?

Did you already have a look at the related troubleshooting support article?

https://support.plex.tv/articles/200931138-troubleshooting-remote-access/

There’s also a guide with some more details on how to troubleshoot remote access issues.

Plex depends on authenticating via their backend. You can configure your setup to minimize this.

You don’t need a Plex Pass membership to access your own server remotely.