You can activate LINQPad on a machine without Internet connectivity if you go through a few extra steps:
Purchase an activation code and try to activate LINQPad - and wait until LINQPad displays an error.
The clipboard will now contain an extended code. (An extended code is automatically copied to the clipboard whenever there is an error activating LINQPad.)
Paste that code into the box on this page: https://www.linqpad.net/licensing/offline.aspx