The instruction manual for your Dehumidifier should provide you with information on what temperature to stop using your Dehumidifier in.

According to EcoAir, their Dehumidifiers stop working in temperatures of 5 Degrees Celsius (5°C) – 41 Degrees Fahrenheit ( 41°F). When the ambient air temperature in the room the dehumidifier is being used in reaches this point, you should turn it off as it may cause it to stop working.

Safety guidelines from my Dehumidifier showing the temperature.

Most modern dehumidifiers have built in defrost modes – when the temperature reaches a certain point the Dehumidifier will periodically enter defrost mode so the compressor is prevented from being damaged.
This normally occurs at 15°C.

The maximum temperature a Dehumidifier should be operated in is 35°C so if the temperature is higher than this, you should not use your appliance!