You can follow the steps below to install .net 3.5 on a Windows Server running 2012R2:
- Insert Windows Server 2012R2 installation media into DVD-rom (Riptide will have to do this for your remote server)
- Follow instructions on this link and as described below https://technet.microsoft.com/en-us/library/dn482071.aspx?f=255&MSPPError=-2147217396
- Open Server Manager, add Roles and Features
- Select .NET Framework 3.5 Features
- On the “confirm installation selections” screen, click on the “specify alternative source path” link at bottom of screen
- Type in d:\sources\sxs
- Install
- Remember to remove the installation media DVD (Riptide will have to do this)