Thanks for helping the donkey out of its problem…
On following just one guideline, the following is up to date …
Although my FP3 is now 1+ year old, thus used for over a year, I just did a full factory reset. Then I followed the guideline, but I got stuck on to sideload the file under “here” in Preinstall step 1.
Whoops, I had USB debugging off and thus Verify apps over USB too, and I was not in recovery mode…