On the https://sd.fqdn.tld/index.php/settings/admin/talk page, you will see
Signaling servers +(Add a new server)
An external signaling server should optionally be used for larger installations. Leave empty to use the internal signaling server. Please note that calls with more than 4 participants without external signaling server, participants can experience connectivity issues and cause high load on participating devices.
So it seems you are with the rest of us hoping to find external signaling servers to bounce off of.
=============================================================
While reading this thread and trying to self-educate via web searches at the same time I see the list of options on Wikipedia is long but doesn’t include some options people mentioned above.
I’m a Ubuntu type base server so I’ll be trying Janus on it soon.
BTW: I’m a bit of a hack admin and the path that led me to this initial post was last night when I went to test screen-sharing from a Ubuntu workstation to a new HP WinX Laptop both behind the NAT Router, I couldn’t connect the video stream. I followed error messages and realized the Windows Defender Firewall was the problem, but that led me to finally RTFM and put my own Turn server on my system. Not too difficult. Should have done it years ago. I suck at my job. 
Now I can connect easily with better performance despite the firewall being back on and differen’t operating systems.