This is the odd part:
on Trisquel's working PC, I have:
* open-ath9k-htc-firmware
* firmware-atheros
* firmware-carl9170
on another Trisquel that's not working with this Wi-Fi device, I have none of
these installed.
On the Debian ASUS non working, I have installed "firmware-linux-free", which
seems to fit the description of "firmware-carl9170". I installed and did a
reboot, which didn't change a thing.
"open-ath9k-htc-firmware" is nowhere to be found on Debian.
[Interestingly, "firmware-atheros" is listed as non-free for Debian on the
web (from Wheezy to Sid). It's installed by default with Gnome3 on Trisquel,
though I suppose only the name is the same]
Ah: "open-ath9k-htc-firmware" is only available on Sid (and Buster).
So I'll move to Sid and see how it goes.
Many thanks Magic banana :)
