Bosch Duotronic fridge - fridge cold, freezer warm?

Although the upper fridge unit (approx 12 years old) works fine, the freezer has stopped cooling. The 2 compressors kick in simultaneously, apparently linked to the fridge thermostat. The freezer (no automatic defrost) was heavily frosted but although now clear will still not cool. Is this likely a thermostat issue? Also, with 2 compresors, is the freezer cooling system independent of the fridge, or is there air flow between the two units? Wondering if a blocked air vest due to the extensive frost buildup is the problem. Sorry I do not have model number. Thank you for your time.