I spent the whole part of the morning trying everything possible: from setting the hotspot of Astroberry to work in 5 GHz, to connecting the laptop straight to the Pi with an ethernet cable, to connecting Pi and laptop to the home Wi-Fi. Nothing worked. Connections kept dropping, USB ports wouldn't work, everything broken just like last night.
Finally, I gave the above command a try and reverted to the previous Pi firmware. Everything works as a charm, now. My EKOS imaging profile works in all these modes, now: hotspot, direct ethernet, and home Wi-Fi. Even with hotspot at 5 GHz.
Now I have three possible ways to connect while I am outside. I hope at least one of the three works in my next session(s).
Now the question is: why the developers of Raspberry go and break a perfectly functioning firmware to release one that doesn't work?! If it ain't broke, don't fix it - they say...!