OK, fixed this. I should have been typing “opendoor” into the Device Key field. Duh.
Options now save just fine.
Blynk still says that “Your generic board is not in network” and there is no IP address displayed. It all works fine on the web interface. I will use it like that for a while.
I do not like Blynk very much. It seems more use to experimenters than for my wife’s phone. Not a success. It is far too easy to change options that would permanently stop the device from working.