You can do this using PowerShell fairly quickly. If you mount the ISO or insert the media for the version of Window Server 2012 you are running, then use the following commands in PowerShell prompt:

# have to import the module for the cmdlets
Import-Module servermanager

# you can verify it is already installed
(Get-WindowsFeature Net-Framework-Core).Installed

#Install it, using the drive letter of your media
Install-WindowsFeature Net-Framework-Core -source '<drive letter>:\sources\sxs'

Once that is finished you should get some output that shows it was successful or not.

You can do this using PowerShell fairly quickly. If you mount the ISO or insert the media for the version of Window Server 2012 you are running, then use the following commands in an PowerShell prompt (elevated privileges required so open it "As administrator":

# have to import the module for the cmdlets
Import-Module servermanager

# you can verify it is already installed
(Get-WindowsFeature Net-Framework-Core).Installed

#Install it, using the drive letter of your media
Install-WindowsFeature Net-Framework-Core -source '<drive letter>:\sources\sxs'

Once that is finished you should get some output that shows it was successful or not. Overall should look something like this: