Thanks @Becki this has saved me a lot of stress. I’ve managed to do this now, after a quite a few hours of messing.
Just a few notes from me. Make sure when you follow the GitHub steps, you follow them precisely, including pressing execute TWICE.
I’m not sure whether it makes a difference, but yesterday in iTunes I was picking the iPad app and it wasn’t downloading, it was stuck with the message downloading. I tried today and used the iOS version and that worked.
As others have said, make sure Plex is uninstalled on your iPad and or iPhone prior to transfer back. But my airdrop kept failing, so I tried it after I copied it locally to my Mac (rather being on my NAS) and that worked.
There is no prompt or notification on the iPad to say that the Plex .ipa is installing on the device, once the airdrop has completed. So I wasted ages looking what I can do next. But I was looking for something on my iPad, and noticed that Plex had been installed at the end of my app icons. It must have been sat there all the time I was looking for ways to get it to install. Moral of that story? If the airdrop completes, check to see if it’s installed. It probably has been, but in the background.
One last thing to note for the process. Perseverance, it’s worth the effort in the end and I can quite happily enjoy Plex again. Oh boy, what an experience!