Had the app to buy movies from my NAS via the Amazon Fire TV play. But unfortunately was a waste of money. - Had some MKV's tried, almost all without sound - TS which were recorded with the Panasonic recorder plays not the app - My Panosonic DMR BST730 the app can not access advance what problems with any DLNA app on your tablet or smartphone works
Actually, you would have to return the app, but is not (why ????)
Stay then live with my WD TV that plays mercilessly all had nothing what did not.
The purpose of the Amazon Fire TV as a multimedia control center is in need of a reasonable DLNA app I now also doubtful will probably go back because it's at least.