Welcome to the Forum, @cloudntails! 
I’ve had experiences with stubborn IoT devices, including Plugs. Setting up an original Plug last year took a lot longer than it should’ve:
In addition to what you said you’ve already tried, I’d consider two other things:
- Take the Plug close to your main Wi-Fi source (primary router or gateway) and disabe every Wi-Fi radio on your LAN (e.g., if you’re using a mesh system or other wireless access points in addition to your main router or gateway) except the single 2.4 GHz band.
- Turn off mobile data on your phone but leave its Wi-Fi capability on. I know you mentioned using “airplane mode”, but in my experience this disables Wi-Fi and cellular connections, and I don’t think all “airplane modes” are equal. You want to specifically disable (temporarily) any mobile data connection so that the phone is forced to use only the 2.4 GHz signal your home’s Wi-Fi is providing for an Internet connection.
Note that these are just temporary measures. Once you get the Plug back online (or get frustrated and grab a hammer), you can turn your phone’s mobile data back on, re-enable the other radios (including 5 GHz) on your network, and move the Plug somewhere else. That and the post I linked above (which links to yet another post) might provide a few other things to try before you give up and take this thing to an electronics recycler.