My primary advice in situation with stubborn IoT devices is to temporarily turn off all your Wi-Fi network radios except 2.4 GHz and do the same for mobile data on your phone so that you’re forcing your phone to use only the 2.4 GHz network. Typically I also recommend moving the “new” device close to (in the same room as) the Wi-Fi source, but you’ve already indicated that you’re doing that. If you have other wireless access points on your network, then you may want to turn those off while attempting to add this device so that you have only a single active access point during setup. Also, if you’re using something like mixed-mode WPA2/WPA3 security for your Wi-Fi, you might want to limit that to WPA2 only, at least until this device is online.
Your post reminds me of things like Wyze Plug v1 Wont Connect - #2 by Crease, and that post links back to some older posts where I’ve described my own frustration and troubleshooting while trying to get a 2019 Plug reconnected.