I just changed phone and my Plex app didn’t get activated automatically. I have a purchased activation on Android a while ago and it work great on my last phone, but the button to restore previous purchase isn’t even there, only, Get Plex Pass or Activate through google play.
I don’t have an Android phone at hand… according to the related support article, you should see a paragraph of text at the bottom of the activation page: If you already purchased this app, tap here to restore your purchase