If for whatever reason you need to install .Net Framework 3.5 on a Windows 10 device that is not connected to the internet you can by installing it from the original Windows 10 image using Powershell or Command Prompt.
In my organization our devices are restricted when it comes to web connectivity and all of our Windows Updates are managed by WSUS, so when we need to install .Net Framework 3.5 we use the below method using either Powershell or Command Prompt.
There are two commands that can be used but the one that I find most effective is the following:
DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:D:\sources\sxs
To begin open Powershell but before you type or paste the above command make sure that the original Windows 10 image location corresponds to the drive letter /Source:D: then hit enter.
This is how it should look before you press enter to run the command.
Press enter to run the command.
After a couple of seconds the installation should complete and .Net Framework 3.5 will be installed.
It is best practise to reboot the device after installing .Net Framework 3.5.
The other command which also works can be foudn below, although I have had instances of this failing so it should only be used if the other command fails.
C:\WINDOWS\system32\DISM.EXE /Online /Add-Capability /CapabilityName:NetFx3~~~~ /Source:D:\sources\sxs