It looked like the firmware wasn't flashed correctly in the factory. What I did was to flash a bootloader using an Arduino compatible board and to flash the Marlin software e.g. according to videos from this guy.
Next, the menu wasn't displayed correctly so I added, in the
Configuration.h the following lines:
#define ST7920_DELAY_1 DELAY_NS(63)
#define ST7920_DELAY_2 DELAY_NS(100)
#define ST7920_DELAY_3 DELAY_NS(125)
Now the display is working fine, and the Marlin firmware can be directly flashed from the computer because the bootloader remains there.
I also had a problem with the heating bed which was heating very slow and I had to raise consistently the
TEMP_BED_RESIDENCY_TIME and decrease
TEMP_BED_HYSTERESIS to 1.