*sigh* so looks like #CoTurn does not really want you to use it with #eJabberd ( see https://landchad.net/coturn/ ).
In addition, dynamic FTTH external IP addresses are apparently not simpatico with the required TURN/STUN setup - seems you need all kinds of haxery to detect external IP changes, modify the ejabberd config, and then RESTART the beast, cause apparently reload_config is not currently enough.
FWIW, as far as I could glean, CoTurn would also need some massaging when the external IP changes.
Disclaimer: I might be missing information still!
BUT damn this sucks.
The whole entire point of STUN/TURN afaik is to help a client figure out their external situation.
In my mind, that INCLUDES situations where external IP is fluid. Seems such an obvious thing.
But apparently no. These things seems to come from a time where it as assumed the server would have a static IP.
Dammit.
And it seems the current devs running ejabberd has the same mindset.