This issue occurs as the Home Theatre is handshaking a 4K signal, so the STB delivers this. As the TV is not 4K and he home theatre only passes through 4K (it does not downscale to HD) this results in no video or usually a message displayed from the home theatre.
A fix would be to buy a cheap HD switch that doesn’t support 4K and plug the STB into this, then into the home theatre. This SHOULD work.
i hope this helps.
I work in Media Operations.
Please note that I am not a part of the social media team and help out on my own time where possible.