Hi,
I noticed the update version 4.18.2001.112 for Windows Defender it not latest version, we could download update package KB4052623 to USB disk on another network connection computer, then install this package on this offline computer to update Windows Defender to Version 4.18.2006.10.
Here is the download link:
https://www.catalog.update.microsoft.com/Search.aspx?q=4.18.2006.10
Bests,