enx0050b6c0f7f3 is in fact my docked ethernet. I solved the problem when I discovered it was specified ‘unmanaged’ for NetworkManager. I added a note in my original post.

Actually, it says:

Loaded: loaded (/usr/lib/systemd/system/NetworkManager.service;enabled; preset: enabled) 
Active:active](Loaded: loaded (/usr/lib/systemd/system/](Loaded: loaded (/usr/lib/systemd/system/NetworkManager.service;enabled; preset: enabled) 

And wifi works OK.

journalctl -xeu NetworkManager | grep enx0 gives:

Oct 14 12:43:11 tpkde NetworkManager[979]: <info>  [1760442191.5289] device (enx0050b6c0f7f3): carrier: link connected
Oct 14 12:44:22 tpkde NetworkManager[9415]: <info>  [1760442262.6582] ifupdown: guessed connection type (enx0050b6c0f7f3) = 802-3-ethernet
Oct 14 12:44:22 tpkde NetworkManager[9415]: <info>  [1760442262.6670] device (enx0050b6c0f7f3): carrier: link connected
Oct 14 12:44:22 tpkde NetworkManager[9415]: <info>  [1760442262.6677] manager: (enx0050b6c0f7f3): new Ethernet device (/org/freedesktop/NetworkManager/Devices/3)

It is a mystery why ethernet works as expected in a live USB session, but it doesn’t in the installed setup even though it is detected and there is no error message.

Interesting. Just tested, and my ethernet works in a live USB session, so it doesn’t look like it is locked by Windows. Also, I have been properly shutting down, only logging into debian for a couple of weeks but the problem persists.

Good idea. With the live USB my ethernet works fine right from the start. So, it’s not hardware or windows. The one difference I see between the live USB and my current setup is that in the live USB session sudo systemctl status NetworkManager.service returns also the line below which is missing when I execute the command in my actual setup:

audit: op="statistics" interface="enx0050b6cOf7f3" if index=3 args="2000" pid= 1957 uid=1000 result="success"

But Info Center in KDE Plasma lists “enx0050b6cOf7f3” as in my original post.

So, ethernet hardware functions, it works as expected with live USB and Windows. In the debian setup it is detected with an inet address, but NetworkManager ignores it.

DHCP also works – my wifi connection in this debian setup works, as do several devices connected to wifi and ethernet.