1-. Downloaded package firmware-realtek from package.debian.org.
2-. Booted Debian Live.
3-. Copied package from hard disk to the desktop (apt complains when
I load it directly).
4-. Executed: sudo apt install firmware-realtek
5-. Executed: sudo depmod -a
6-. Executed: sudo modprobe r8712u
Lets see if driver was able to run your card:
Check output of
$ lspci
$ iw dev
$ ip link show
If everything is ok, check if your card can see any network
$ iw dev [YOUR_CARD_DEVICE] scan
(I do not remember exact syntax, try "man iw" or "iw help")