Windows 10 from 1809 onward, pulls optional features from WSUS so will show no available updates unless they are configured.
To get around this you can configure a PC to search online for available optional features…
Run “gpedit.msc” to edit your local computer policy The setting in question is: Computer Configuration\Administrative Templates\System\Specify settings for optional component installation and component repair
My local policy seems to have defaulted to “Disabled” – after changing it to “Enabled” and selecting the checkbox labeled “Download repair content and optional features directly from Windows Update instead of Windows Server Update Services (WSUS)” the RSAT tools installed for me.
Then you can powershell to get the updates you need e.g.:
Add-WindowsCapability –online –Name “Rsat.ActiveDirectory.DS-LDS.Tools~~~~0.0.1.0”
Check out this link for lots more options!