Help with LAN installation

Good Morning
First of all thank you very much for your help.
I want to install jitsi on one of my lan’s servers. I do it on a ubuntu 18.04 that is running as a virtual machine on the server with QEMU / KVM
Since the intention is to have this installed for a moment only for the local area network, the machine is not visible from the outside. I have configured the machine in the domain and used his data for the installation: mymachine.interno.mydomain.es
I followed the installation step by step, except for the firewall, which did not activate it at the moment because it blocked the DNS discovery of the machine on the network.
The system apparently works, but when I walk into a room it reconnects me at 35 seconds.
In addition, only one user enters the room at a time. If I enter the room “test” with a user, it is created. Afterward, I enter the same room from another computer and appear alone in the room as well.
Can you help me?