VLC and RTSP behind NAT

[patpro]

Hello,

I'm trying to read RTSP video streams (from mafreebox.freebox.fr) on VLC 3.0.x but every device on my LAN sits behind an internet gateway doing NAT and firewalling. The gateway is a FreeBSD 12.x box running pf as the firewall.
I can read RTSP streams on a particular device on the LAN if I setup a dedicated port redirection in the gateway firewall for the device's IP address and setup fixed port in device's VLC. Obviously it's not a good solution, unless I setup up a different port redirect for every device and I setup every device with a fixed IP address and a different fixed port in their VLC.
I've tried and used osrtspproxy on the FreeBSD gateway, but I couldn't make this work.
I've also tried miniupnpd on the FreeBSD gateway. This software should have allowed each VLC client to create their own ad-hoc port redirection rules in the gateway firewall via UPNP but it didn't work at all (from my Mac, didn't test from a Windows or smartphone).

Any idea?
thanks