Distance 
Christ I might have the air traffic police after me if I went that far up lol
I would suggest you stop what you’re doing. No amount of tinkering will get you further when flying over urban areas from your back garden, clearly you cannot see the craft and it cannot see you which is why it is losing signal. Trying to push it any further, at best, risks the possibility of losing the craft altogether, at worst, it comes down and injures somebody or damages their property.
Get yourself out into an open area or over the sea. I have had the Mavic Air as far as 3000ft over the sea without a problem at 200ft height. Worst case scenario I lose the craft without injury or damage to others. Once you get beyond 2000ft you can neither see it or hear it which is a scary prospect on its own but to do that over urban areas is playing with fire.
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