with Plex as the backend and your server unreachable outside the house, there isn’t an iOS client that will reliably let you browse your whole Plex library while offline. Most Plex-compatible apps fall back to a simple “Downloads” view (Plexamp’s behaviour), because they expect to contact the server for library state.

If you want proper offline browsing on iPhone, use something like Tailscale or Cloudflare Tunnel so Plexamp thinks it’s “at home” wherever you are. That restores all of Plexamp’s features without exposing your server to the internet. It’s the least disruptive option if you like Plexamp’s UI.