If the unit was running around 0° and then goes down to -20°, you may have a temperature sensor issue with the unit. I have attached a copy of the thermistor chart to test the sensor. You will need a multi-meter to test it. The thermistor is located in the vent of the freezer assembly. If it is bad, you will need to order the assembly for the thermistor. Here is the part number for that as well.
Part number: AP4169102