Often when I use FaceTime it will run over mobile data, even when a good working Wi-Fi is available. And this always with the same Wi-Fi
Wi-Fi Assist is turned off as recommended in this Answer.
At the moment I'm turning on Airplane Mode, to definitely use Wi-Fi. Works always without any problems and the connection is always great! Of course I forget it from time to time, so it uses my mobile data....
Turning off the cellular for FaceTime is also no option. I live in another country than the rest of my family. So they usually call me over FaceTime and I want to be always available.
This whole thing started some months ago, with some update. I didn't noticed it immediately.
Maybe I'm missing out on something easy?