Questions about setting up my own stun/turn server

I have some question on how to setup my own stun/turn server. I already did setup my own coturn server for Nextcloud using this guide:

My current setup is that I have the coturn server in my local network and I forwarded the port from my pfSense firewall. My Jitsi server is also on the same server as the coturn server.

I did test my coturn server using this link:

If I do the test from outside my network over the internet the coturn server returns the external ip address of the client. But if I do the same test from the internal network I get no response from the coturn server.

My question is now, is it even possible to use a coturn server in the same network for this or does the server has to be in the internet to determine the external ip address? And if this is true I guess I could just use the server as a turn server and use a free stun server for the ip lookup?

I think that’s exactly how Jitsi-meet is setup in Debian packaging, the server is set in the for javabridge2