Jitsi: disconnection when 3 or more people connect or using Browser / PC

We have some troubles for an on-prem installation, an internal Vm (debian10 based + nginx) used as server. There aren’t resource issues (CPU, RAM and NET are ok) . We can add 3/4 or more people but from the fourth person, added / new people cannot be displayed (webcam). Some times, people using PC (in some private LAN) disconnect and reconnect continuisly. What I can check?

Are the Jitsi server and all clients in the same network block?
Do all clients access to UDP/10000 of the Jitsi server?