How to install Broadcom wireless drivers offline?

You need to install wireless driver.

Insert your Ubuntu installation disk or a flash drive and copy these files from the installation disk to your Home directory:

pool/main/d/dkms/dkms_XXXXX.deb
pool/restricted/b/bcmwl/bcmwl-kernel-source_XXXXX.deb

Then run the following in terminal:

sudo dpkg -i *.deb

If you have UEFI BIOS, you need to disable Secure Boot to load the driver.

18.04 update:

The solution won't work on 18.04, because build-essential and its dependencies are not installed by default. The dpkg command will complain that dependencies are not installed.

You can manually find the required packages on the installation media ("pool" directory sorted by alphabet) and install them the same way as the dkms package. I hope nothing is missing there.