NumberSync

I've been trying to activate NumerSync for 5 days.  i bought an esim, and i would like to receive and make calls on my galaxy watch 3 when i'm away from my cell phone, or when i don't have it.  in the At&t app when I go to manage/device and go to manage number Sync it doesn't move!  the screen is blinking, and then back to the same place!