Problem: DLNA server (Plex) visible in the network. The content is not available.
Solution: disable authorization in DLNA server.

Go to Settings - Network - "List of IP addresses and networks that are allowed without auth" (for Plex server) - enter 192.168.1.0/24 if you using 192.168.1.1-254 network.

Enjoy VR!

3 months later

stabbicus Dunno about iPhone, but on google app store there are apps to test network protocols (SMB FTP HTTP DLNA ...)
Maybe check if it's actually on and detectable.