2 users working fine 3rd user comes in and all Audio video loss

I have installed jitsi as per guide on ubuntu 18.04 server. now my server doesn’t have internet connectivity. all my users are on same LAN without internet connectivity.
this setup is working fine with 2 users only but as soon as 3rd user joins all users lost audio and video connectivity .
what could be the problem. I don’t need connect from outside my LAN. all users are inside my LAN on same vlan with no firewall between local jitsi server and users.