I had the same problem with my Gen 2 thermostat and 6.2-22. I ran all the interactive trouble shooting techniques. Nothing. The thermostat didn't see any wifi signals and in a resort town I normally see at least 10 of varying strength.
I removed the thermostat from my Nest app. Then I reset the thermostat to factory reset. After that was done I did a hard reboot by holding down the bottom of the ring for 10 seconds. Display went black and after about 30 seconds started back up. Once it was done I reinstalled the thermostat back onto my Nest app. I entered the pin I got off the thermostat. Done. It works! I noted that the 6.2-22 version was gone and it reverted to 5.xx (didn't write it down).
I suspect the hard reboot of the thermostat along with removing and reloading on the app did the trick.