https://askubuntu.com/questions/730799/installing-firmware-b43-installer-offline
You need to install firmware-b43-installer.
If you do not have an alternative internet connection, do it this way.
If you installed bcmwl-kernel-source, then remove it.
sudo apt-get purge bcmwl-kernel-source
Download these files http://mirrors.kernel.org/ubuntu/pool/main/b/b43-fwcutter/b43-fwcutter_015-9_amd64.deb http://www.lwfinger.com/b43-firmware/broadcom-wl-5.100.138.tar.bz2
Copy them to your Ubuntu Home folder.
Run in terminal
sudo dpkg -i b43-fwcutter_015-9_amd64.deb
tar xfvj broadcom-wl-5.100.138.tar.bz2
sudo b43-fwcutter -w /lib/firmware broadcom-wl-5.100.138/linux/wl_apsta.o
Reboot or run:
sudo modprobe b43
http://www.lwfinger.com/b43-firmware/broadcom-wl-5.100.138.tar.bz2 doesn't exist anymore, is there a mirror or something ?