Granted the issue was identified years ago, but happened to me APRIL 2021. I fixed it. Not covered in the manual. But each step I provide are covered in the manual. My condition after a software upgrade was automatically made.
Steps:
On the watch go into settings> networks> and delete the one (or all of them) you are trying to sync (on;ine owner's manual is clear and will get you there or close enough that your kids can guess the next step)
With the watch on, use your smart phone and the "Garmin Connect APP", not "ConnectIQ", and add a network (this is covered in the manual and you did this originally when you set up the WiFI)
That's it- should work and you're done
My watch could not find any networks even though they were listed with the correct passwords. So by deleting all of the networks, the "smartwatch" was now dumb enough to start looking. It re-found the networks, I entered the password and now I am ready for another software upgrade! :-)