Does some know if I´m doing something wrong? Or do you have any advise please?
You are probably not doing anything wrong, this is a very common problem with all Bitmain 17 series gears, these boards break really easily and it's hard to fix them, if your supplier really fixed it then they probably didn't solder/glue the heatsink well and it lost contact again, of course, this assumes he is honest in the first place.
There isn't something you can do about it unless you are familiar with electronics and have the tools needed, ask him for a refund or perhaps send it to a repair center nearby, where are you located?
Thanks a lot for your advise. The issue I have is I´m in Germany and can´t find anyone here or in another European country who can repair.