Trial version is not the issue, it is definitely fully functional.

Being on separate drives could well be the problem. When reaper starts up it scans a set list of directories looking for plugins, if kontakt is not within the list of directories being scanned, it’s not going to be found.

I’m not on my music computer at the moment so I can’t tell you exactly, but there is a setting in reaper where it shows you what directories are being scanned, and also allows you to add a new directory.