Weird. It has always worked perfectly fine for me. You must have something interesting going in in your setup.

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If you enable the “remote access” in Plex you are essentially port forwarding you server to the internet using UPnP (by default. You can also port forward manually if you’d like).

It’s indeed a point to point connection but a point to point connection the same way your connection to normal websites are point to point.

If you knew the public IP of anyone that’s using Plex you can likely go to [IP]:[Random PORT] and access their server. You still need to login though.

Source: My own tests and https://support.plex.tv/articles/200931138-troubleshooting-remote-access/

No, that should work straight out of the box. Maybe you have some network configuration that stops that, like a firewall.

The quality was probably bad because you were routed through Plex Relay services which have a bandwidth limit. It is honestly quite a nice free service because it means it will work pretty much regardless how your network is setup but the quality will be bad. If you want to directly connect to your server you need a public IP so CGNAT won’t do you might also have to open some ports.

If all you want is a local media server. It’s very easy.

You pretty much just have to install Plex or Jellyfin, setup a “library” in the software.

You usually set up one library for movies and one for TV shows. You then point these libraries to their respective folders on your hard drive and assuming you have some half decent organized media with proper naming it usually just works.

Plex doesn’t have automatic subtitles per say but mostly Plex players allow you to download new subtitles from the player. I don’t know about Jellyfin.

If you want to have external access it’s a bit harder if you use jellyfin as you will have to setup a reverse proxy but I’m guessing that there are a lot of guides for that online. Plex should work for external access out of the box assuming you have a public IP, and even if you don’t you can use their automatic relay services to get it to work anyway although in very low quality.

Proper naming is honestly the hardest part but that’s very dependent on how much existing media you have and how the naming is today. Luckily Plex and Jellyfin are fairly good at recognizing and finding media with subpar namin (you should still fix the naming to comply with the documentation)

If you want to have automatic torrent downloads, fully automatic subtitles and all that it’s quite some work to set it up properly and have it working without any input from you. If you want to tackle it (or are just curious), I recommend checking out https://trash-guides.info/