Problem with downloading Windows updates via WSUS due to DualScan

Sometimes you may encounter the problem of installing Windows updates through the WSUS server if you are not connected to the Internet. The likely reason is the DualScan mechanism, which is such a combination of settings in Windows 10 1607 and higher that clients start to ignore the settings of the local WSUS server, while simultaneously turning to scan for new updates to external Windows Update servers.

You can disable DualScan using Group Policy: Computer Configuration->Administrative Templates->Windows Components->Windows Update->Do not allow update defferal policies to cause scans against Windows Update.

Or add the following parameter to the registry:

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate]
"DisableDualScan"=dword:00000001