External monitor remains dark with MB Air 2020 M1 HDMI

Sonoma 14.4.1

My 31" AOC monitor works perfectly with my wife's 2020 Intel MB Air. I use the same cable and HDMI adaptor to connect to my 2020 MB Air M1. What happens is that the external remains black while my screen reverts to the log-in screen picture with a grayed-out menu bar at the top, no cursor. I can't get to the Display Settings, indeed, I can't do anything until I disconnect the HDMI adaptor. Obviously, the Air is detecting something so it's not a complete non-connect issue. I've spent a lot of time googling this but I can't find any solutions or workarounds. Any help or advice appreciated.

I located this page:

If you can’t see your Mac desktop on an external display - Apple Support

But it doesn't address my problem.