Your post reminds me of the hoops I had to jump through last year to get one of these things back online:
One thing I failed to mention there was that I took the Plug to the same room as my Wi-Fi gateway as an attempt to improve its chances of connecting once I got past the step where the phone does the hand-off from the Plug’s own local Wi-Fi. I also turned off my phone’s mobile data so that it would be forced to use the gateway’s 2.4 GHz Wi-Fi for its Internet connection.
Additionally, I may have tried using a phone with an older Android version when a newer one wasn’t getting me connected. If you have an older iPhone, then that might be worth trying if other things fail. I have limited experience with iOS and haven’t used that for >10 years, so I won’t be able to answer questions specific to that platform.