LAN vs. WAN

I have my own Jitsi installation which seemed to work as expected, with one exception : Some clients, connecting from WAN, see after a few seconds the well-known error : ‘Something went wrong …’.
Unfortunately, the error is not really specified, leaving me guessing, since the logfiles do not tell me much about it.
For one client, this happens all the time.
For other clients, the mileage varies.
For some other clients, it never happens.

As soon as these clients connect to my VPN first, everything is fine.
The following ports are forwarded : 80, 443, and a mediaport ( 30000 ).
Since the above behavior could point to a port problem, is there anything I might have left out ?
What about the port 4443 I sometimes read about ?

Does videobridge listen UDP/30000?
Normally it works on UDP/10000

As far as I can see, it does.