How to fix external unit not working

I have a K40911 ELVOX Door entry system - Stand-alone KITS - Kits - Two-family entry system with an external unit with two buttons connected to two displays. The system worked for 3 years and now the outside unit seems to have no power anymore. The two displays work just fine. I have checked online and it seems to be a known issue with the product though Vimar refuses to acknowledge it and to replace their products. They also refused to apply the 5-year warranty in my case and I am now stuck with a system that doesn't work.

There are no tutorials to be found online at the moment.

Does anyone know how to fix the issue with the motherboard in the external unit?