Part number: AP4303701
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In the FAQ it says that the reset temperature of the thermostat is 15.6 degrees so perhaps they are just not cold enough.
The working temperature of a freezer should be 0 to 5 degrees F.
Check your meter.
Shorting the meter leads together should give 0 ohms.
Try a higher meter scale.
I personally do not like immersing thermostats in water.
Get a drop of water into the thermostat and it is toast.