Variable freezer temperature, variable and too hot fridge

Hi, I have a Miele KDN12823S combined freezer-fridge. Freezer at bottom, fridge at top. Problem1: too wide temperature swings in freezer. Temperature should be around -20 degrees Celsius. However compressor does not start before -10 degrees Celsius is reached, and then draws it down to -30 degrees Celsius. Problem2: temperature in fridge is set to 5 degrees Celsius, but is warmer, around 8 degrees. It also varies a lot, between 6 and 10. Appliance has been turned off for 48 hours, so is defrosted. Temperature sensor in freezer has been changed, so works. I have a network connected TempStick in freezer that reports temperature every 30min, so have a temperature graph of the freezer. As far as I understand, both fridge and freezer has its own evaporator, which means there is no damper control. Freezer fan has been tested and works. What could be the problem?