The frequencies are just such a thing... Of course, it depends on which FM radio stations are currently being received strongly at your particular location and which are only weak. The better the reception of an FM radio station, the worse your car radio will be able to receive the signal of your DAB device on this frequency. The transmitting power of the device is rather low.
You would therefore have to check on which frequencies in your location the radio programmes are not received at all or only very poorly. And unfortunately this changes again and again when the car moves. Therefore, it will always be a bit tedious with this solution... ...until the FM stations are switched off. Then you will certainly have much fewer problems.