I have a Galaxy S8 and have the same problem. When I try to activate wifi calling after a couple of minutes get the error message "Unable to activate WiFi calling at this time, please try again later". Later still get the same message.
I tried to follow the instructions for turning on Advanced Calling but from settings>connections that option is not listed. I tired settings>connections>more connection settings but not there either.
Any suggestions on how to enable wifi calling on my phone?