Hi Pavlos and welcome to the Mattermost forums!
Is there any firewall in front of your Hetzner VPS? Something that maybe blocks incoming UDP traffic? You need to verify that you can connect from a client to your server on UDP port 8443. You can do that using the nc command line utility f.ex.
To do that, stop the calls plugin, the local port 8443 on your Hetzner VPS should be unoccupied again.
On the VPS, run the following command:
nc -l -u -p 8443
On a client, run:
nc -u <public_IP> 8443
You can then start typing in any of the two commands and the text should appear on both ends when you hit return. If it does not, your connection does not work which will also explain the behaviour you’re experiencing here.