I’ve had Plex for years and I’ve recently found out you do Plexamp and I wanted in on that. I have downloaded the iOS app, the Android app and the MacOS app and none of these will find my server, either locally or remotely. I have never had a problem with accessing my server through the main Plex app.
Even though both Plex and Plex Dash (which I have literally installed on my phone in the last 5 minutes) can see my server locally and remotely I have ensured my router has the right port forwarding set up, my server has the correct public port, etc.
How far fo you get with the app?
Are you able to sign in with your account?
Are you getting to the point where you‘re asked to select the default library?
Especially on iOS, you might need to enable local network access for Plexamp in the iOS system settings.
TL;DR You need to have a music library on your media server recognised by Plex, i.e. your music needs to be added to the main Plex libraries.
So I have my media library set up with movies and TV shows, but on my server I have a music folder. In my mind I thought Plexamp would create its own music library separate from the main Plex setup. Turns out you need a music library in Plex for Plexamp to work.
So for future reference, add your music to your main Plex server and then it can be found by Plexamp. Not sure if I’m just stupid or this needs to be explicitly stated somewhere.
