Another from Matt the Quick:
By default, WSUS does not have .NET 3.5 available to install. This creates issues when .NET 3.5 is needed after a machine has joined the domain. Below is the workaround steps to install .NET 3.5 using the operating system ISO and not Windows Update or WSUS.
- Mount the ISO of the operating system. The Server 2016 ISO is known to work for this.
- Open an Administrative Command Prompt and run the following command:
dism /online /enable-feature /featurename:netfx3 /all /source:D:\sources\sxs /limitaccess
NOTE: The Source flag will have to be changed to reflect the Sources\sxs folder on your installation media. This is likely going to be the D drive (as in the example above), but it’s possible your results may differ slightly. - Reboot machine.