Can't set the thermostat lower than 18 degrees

Hi, I have installed the the Smart Modulating Thermostat to my modulating boiler, but I can't set it lower than 18°C. According to the specs I should be able to set it to as low as 5°C.

It doesn't matter if I use the device itself, the app or the Apple Home interface.

I wonder what is happening here, because this way the thermostat is useless for me.

Regards,
Melle