How to install ethercat without internet?

Is your workshop anywhere near your house/internet access?

Long network cable.

Or get a wifi dongle and connect to wifi.
Or wifi dongle and use a phone as a mobile hotspot.

The hardest way is to try and do all this with no internet on the LinuxCNC computer at any point. You don't need to have internet access all the time.

You can install one off my deb packages from a usb.
This file contains a ethercat package.

But if you need other dependencies, you can do : sudo apt-get download ... for them.
But you really need to check this carefully because some deps, need other deps etc.
So be sure you have the complete list of deps downloaded. I did such things before, it works.

Easyest way is to install a linux with ethercat already on it. Something like this.