En attendant le patch du serveur “Synapse”, une solution qui permet d’avoir un DNS resolver en IPV4 à DNS resolver : how to add permanently an IPV4 server?
Avec ce changement, la fédération entre serveur Yunohost fonctionne (chat, appels, …).
Je n’ai cependant pas la confirmation des dev’ de Yunohost que ma solution n’a pas de conséquence sur le fonctionnement et la sécurité de mon serveur Yunohost…
[Edit : I wrote in French indeed, sorry for that… translation below + an addition]
Waiting for the patch of the Synapse Server, a solution that allows (in my case) to have a DNS resolver working with IPV4 at DNS resolver : how to add permanently an IPV4 server?
With this change, federation works (chat, calls,…)
However, I am waiting for the confirmation of Yunohost dev that my proposed solution has no consequences on the running and the security of Yunohost server.
In addition, I added a turn server to facilitate voice/video calls (instruction at https://github.com/matrix-org/synapse/blob/master/docs/turn-howto.rst).
Everything seems working ! 
What a great chat / voice / video solution, decentralized, secured, and free software !