apple tv 4k audio but black screen

When we first connected the device, it worked fine but then we found that when an episode finished and it moved onto the next one, we had audio but no video. My understanding is that if there is not enough speed (and apple recommend minimum 25mbps and we only get 18-20 mbps) that it will adjust the quality to work with the speed. It looks as if it is not adjusting the quality just refusing to receive the video. After multiple attempts we did get the video back but it took so long that we put our old apply tv unit back on which works perfectly but won't take apps and as our Sony tv now won't take apps either, and not all UK channels will allow airplay (like All4) we cannot watch their programming which is why we bought a new apple tv device. Is there a setting or a work around for this?