Some apps will support 24Hz(movie frame rate) output on certain devices set to match frame rate. But, not all will do so and there will be issues when trying to play 4K HDR 60Hz in this particular case. You can try to set the Fire Cube to output 4K @30Hz using the existing cable to see if the reduced bandwidth needed for output will get you a a 4K HDR signal with the TV in "Enhanced" mode.