For Debian Buster, with internet access, a mix of the two other response is what you need to go :

You need to enable non-free first: edit /etc/apt/sources.list, and at the end of lines ending with main, add contrib non-free. You'll end up with something like

deb http://ftp.fr.debian.org/debian buster main contrib non-free 

you can now install

sudo apt install firmware-linux-nonfree 

and eventually  :

sudo apt install firmware-realtek 

For Debian Buster, with internet access, a mix of the two other response is what you need to go :

You need to enable non-free first: edit /etc/apt/sources.list, and at the end of lines ending with main, add contrib non-free. You'll end up with something like

deb http://ftp.fr.debian.org/debian buster main contrib non-free 

Once that's saved, update the indexes with the new package lists:

sudo apt update 

You can now install

sudo apt install firmware-linux-nonfree 

and eventually:

sudo apt install firmware-realtek