Hold the device in your right hand, take a good stance, open the window and then move the device as far away as possible.
Seriously, extend the antenna, but that's usually not enough. It is also important to know where the nearest transmitter mast is:
https://www.bakom.admin.ch/bakom...

Extend the antenna fully. Place the unit in front of an open window or in another location where reception is better.
You can check the DAB coverage here:
https://www.broadcast.ch/de...

If nothing helps, the radio is defective.