Bluetooth mouse won't connect, after turning Bluetooth off it won't turn back on
create a USB drive with windows support software using Bootcamp Assistant (mac os)
go to windows and install that support software from the USB drive
go back to macOS
remove ALL Bluetooth devices from mac os
set windows as the startup drive
restart and open windows (click mouse and keyboard when you see windows logo)
go to Bluetooth settings and try to connect 


worked for me ...for a moment...then I lost it when I set macOS as startup drive.