All the steps described by @SteeMan are fine. The details are the particularities of each TV BOX, in case of Brazil, the box are the low quality and price (the cheapest box of the market). I had many difficulties with a RUPA A9 Diamond with S912. Initially, the problem was the network, so I created new dtb´s files based in a post of someone. 
All the steps are described here:  linux_variety/notes.txt at master · claudiosa/linux_variety · GitHub

some steps to generate dtb and dts
 

Later, the sound was not working, fortunately I tried some others dtb without the procedure describe above, i.e. without network.
Fortunately, the first experiment with "MeCool KIII Pro" dtb the sound works.
So, I modified dts to ethernet become stable... and everything is working cool.

I installed ICEWM and ssmd to manager desktop windows. My experiments are in Youtube, unfortunately in PORTUGE, but put translation in Englsh in subtitles that works fine.
here some links:

first part of video

2nd part of this video

3rd. part of this video

 

 

 

 

 

 

Some  important details  by me....