The display darkens when the room is darkened, if it always remains dark, there may be something in front of the sensor so that no light reaches the sensor, it is more likely that there is a defect. If the display remains completely dark, it is probably defective.

What do you mean by "the display stays dark"? Do you mean "standby mode"?
With regard to the display brightness, there are various setting options in the menu (see manual on the Galaxus product page (Downloads) / page 15 --> Display settings).
The menu settings are not quite logical, but with a little trial and error you can find the desired brightness settings. In any case, it worked for me ...

Hello, if you press any button on top of the device, does the brightness change? If not, as I said below, it's probably faulty. For the record, I had to change mine because it couldn't pick up any radio channels, and the new device works perfectly.