Thank you for this clarification.
My config:
Plex Server on NAS Synology DS214Play
Manual port on 34000
Searching for DLNA servers and clients disabled
Router: port 34000 forwarded to the NAS
Security connection but not forced.
…and it doesn’t work from the 2 TV boxes installed on the same LAN.
On the other hand, no problem from a 3G mobile, from the WAN, etc…
Grrrrrrrrrrrrrrr.