If you don’t mind, could we please continue the conversation in English to avoid any mistranslations and therefore miscommunication?
If the device firmware does not upgrade after the bootloader process, this could indicate a flash-memory issue, which is hardware-related, and requires physical repairs.
In short, repairs can only be done by us to products which have been purchased from us & have their warranty still valid. If it has been purchased from a 3rd party, then contacting them would be the solution here, but since they are not responsive/don’t exist anymore, all I can really suggest is trying to find another repair shop to look into the device.
I’d also like to clarify, when you say that the firmware fails to install, does it simply stay the same as it was before attempting to do the process, or are you met with some type of error message?