Do this:
wget http://ftp.us.debian.org/debian/pool/non-free/f/firmware-nonfree/firmware-iwlwifi_20190114-2_all.deb
sudo dpkg -i firmware-iwlwifi_20190114-2_all.deb
If you don’t have internet access to use the wget command, then download this package:
https://packages.debian.org/buster/all/firmware-iwlwifi/download
Save it on a flash drive and put in into your PC and then either use the dpkg -i command as shown above or right click on it and select the GDebi package installer from the popup menu to install it (if you have gdebi installed).