Install 18.04 server without network connection

You can install Ubuntu 18.04 without an internet connection when using the "standard" ISO. (not the live server)

You can download the standard one here: http://cdimage.ubuntu.com/ubuntu/releases/18.04/release/

Originally downloaded the Ubuntu 18.04 LTS Live ISO. The installer recognized my wifi network card but could not sign in to the WIFI network and therefore the installation would not let me continue. Following a suggestion in this thread downloaded the ubuntu-18.04.1-server-amd64.iso and installed it on a USB drive. The installer on this ISO recognized my WiFi card and allowed me to login to the WiFi network. The ISO is located in Ubuntu.com -> Download -> Ubuntu Server -> Use the Traditional installer -> Download the Alternate Installer -> 64-bit PC (AMD64) server install image.

This is a known bug in the new installer. Until it is fixed, you can use the traditional installer as dhelios points out.