What box? An android TV box is too general... Have you tried installing the latest KODI version? Also when u say, KODI cannot ply the movie files, what errors message pops or after navigating to the file in KODI, click to play does nothing? Try other external players, might work as most boxes don't play nice with KODI.

Go to Kodi > System > Setting > Video > Acceleration, change to hardware acceleration and see how.

If not, play around the settings there.

There are addons for poster/fanart that you can install to automatically fetch the necessary files.

However, I prefer to choose my movie posters.

Copy the poster jpg file to the folder of the movie and rename it poster.jpg. The folder will appear in Kodi as a poster. Make another copy of poster.jpg and rename it name_of_movie_file.tbn and the movie file will appear as a thumbnail.

A little bit of extra work, but I organize my movie collection in exactly the way I want.

You can use MediaELch or Ember Media Manager to organize your library and set scraping in KODI to local info. I prefer Ember.