The wireless networks list show nothing after trying Xubuntu 18.04.

This command did the trick on Live CD.

• sudo apt install bcmwl-kernel-source

But after installing Xubuntu 18.04 to the disk, the same problem happened and the above command didn't work.

So need to download the required packages from the internet...
Or it is also possible to use Live CD as local repository without internet.

Using Live CD as Local Reposityory

Edit the sources.list file:

sudo nano /etc/apt/sources.list

And put the following line and save it.

deb file:/media/username/Ubuntu bionic main restricted

Now run the following commands:

• sudo apt-get update --allow-unauthenticated
  
• sudo apt-get install bcmwl-kernel-source

https://www.rubix.nl/blogs/ubuntu-18-04-no-wifi-adapter-found/

download a package and its dependencies to a folder:

apt-get download $(apt-cache depends --recurse --no-recommends --no-suggests --no-conflicts --no-breaks --no-replaces --no-enhances bcmwl-kernel-source | grep "^\w" | sort -u)

sudo apt-cdrom -m -d /media/cdrom add
sudo apt install bcmwl-kernel-source

https://askubuntu.com/questions/1068351/solved-difficulty-installing-bcm43142-offline-for-ubuntu-18-04