networking - Install Broadcom wireless drivers on Ubuntu 18.10 "without internet"

I have a desktop with an Asus Z170-Deluxe motherboard, which has onboard Wifi, and I'm trying to be able to use that to connect to the internet on Ubuntu 18.10.

First I thought it might be a good idea to try with the live USB, and when starting, there was no connection, but after doing A LOT of research, I found out I was able to go to Software & Updates > Additional Drivers, and there I could select this:

Using Broadcom 802.11 Linux STA wireless driver source from bcmwl-kernel-source (proprietary)

and then I could connect to the internet. So I thought the issue is resolved, and I installed Ubuntu 18.10, but after installation when I went to Additional Drivers, nothing was listed there :(

I'm not very familiar with Linux driver stuff, so again I started researching, and I found some that I wanted to try, but almost all of them included doing

sudo apt update
sudo apt install ...

but my problem is that I don't have a connection to the internet to be able to do any of those, and I don't have access to a wired connection.

PS: while I was trying things, I saw this item in the Additional Drivers tab:

Using Broadcom 802.11 Linux STA wireless driver source from bcmwl-kernel-source (open source)

but that doesn't seem to work.

Any help is really appreciated