You could find the answer by searching for this problem with Debian -
it isn't Qubes specific.
Once you know what package you need to provide the firmware for your
wi-fi card, make sure that you install it *in the template* NOT in
sys-net. journalctl -b will probably show you an error in sys-net,
telling you what firmware is lacking.
Some people like to clone the template before installing new drivers in
to the new template.
That's up to you.