To install .net 3.5 please use the following steps: -
Open 'Server Manager
'Click ' 2) Add roles and features
'Click 'Next' four times until the 'Select features' window has highlighted 'Features' on the left.
You'll see the '.NET Framework 3.5 Features' item at the top of the checkbox list with the box unticked. You need to tick the box and click 'Next'
On the Confirmation screen, press the 'Install' button and the framework should install. When the Feature installation has completed, the Close button should enable and you can press it.
That is typically all that is required, but if your software still refuses to use .NET 3.5, then reboot the VPS after the feature installation and retest the software.