Ubuntu:Proprietary network drivers unavailable after install

[peter_b]

[PROBLEM]
When installing Xubuntu 18.04 on a Lenovo Thinkpad S21e, the proprietary broadcom drivers are still required for the WiFi.
The problem is, that although these show up in the "additional drivers" GUI on the live system, but once installed that option is gone

This is especially annoying, since without network connection, fixing network issues is a real hassle.
Even if you have 2nd PC nearby...

The reason is, that the installation source (in my case USB stick) is disabled after install.
Enabling the "cdrom" APT source will cause the driver to show up in the additional drivers GUI...

Driver visible, but cannot be installed (yet).

...but when you want to install it, the progress bar starts - and then stays like that forever.

[SOLUTION]

The reason is, that APT still has the list of packages cached - but doesn't "find" the USB stick when it searches for the cdrom source.
The current workaround/hack is to:

1. Enable the "cdrom" source in APT (it actually is the USB stick, but...)

   

   Enable cdrom APT repository source

   [/b]
2. Mount the USB stick
3. Create a symlink to the USB mountpoint at: /media/cdrom
4. Try installing the driver again now

Here's a related answer by HarlemSquirrel (askubuntu.com).