My OSMC is missing osmc-backup

There are detailed and up-to-date instructions on getting Netflix working in this post:

Note that, for practical purposes, Netflix is limited to 720p playback, and that’s not going to change any time soon.

Once you have Netflix working, you should then be able to install the “Amazon VOD” add-on without too much trouble. Playback there will be limited to 960x540 resolution.