YAY @Alexandre.! Connected to the uncooperative Primary unit with a LAN cable and did it all over again. NOW it updated! AND devices do connect to the Deco nearby. Many thanks for your support and...

@Alexandre. ...end all devices still seem to be connected to the primary Deco (the one that didn't update)

Done, @Alexandre. The two Satellite M9's updated as intended, but the Primary remains on 1.2.

Thanks @Alexandre. It says that it applies for Deco M5 or Deco M9 Plus V2 Mine doesn't say V2, but just Deco M9 Plus. Can I still use the second version?

@Alexandre. Trying again to upload the screenshot. I was in too much of a hurry before.

@Alexandre. Many thanks. As the app tells me that it's up-to-date, I'm at a loss about how to update it anyway. Looking forward to that instruction.