AcuRite Digital Wireless Fridge and Freezer Thermometer with Alarm and Max/Min Temperature $25.56 at Amazon

Digital Thermometers: Includes one refrigerator thermometer and one freezer thermometer that transmit readings to the digital display
Easy-to-Read LCD Display: Comes with magnetic backing for fridge mounting and a keyhole for mounting on a wall
Customizable Temperature Alarms: Be alerted to temperature spikes caused by mistakenly leaving the fridge or freezer door open, or a failing appliance
Multipurpose Silicone Sensor Straps: Can be used with the included suction cups or wrapped around a wire shelf inside the fridge or freezer
Temperature Range: -40 °F to 100 °F (-40 °C to 37 °C)
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This dual-purpose freezer and fridge thermometer manages to be in two places at once. Using two wireless, remote thermometers, communicating with a door- or wall-mounted display, you will be able to be alerted when freezer or refrigerator temperatures are out of preferred range. This thermometer helps you store foods at safe temperatures to prevent food spoilage and avoid related foodborne illnesses. The wireless thermometers provide data to the digital thermometer display to show refrigerator and freezer temperatures and record highs and lows for each environment, as recorded by each remote thermometer. A programmable alarm notifies you audibly and visually when temperatures exceed your set minimum or maximum range. The digital thermometer display has an integrated magnet and keyhole for easy mounting. The digital display requires 2 AAA batteries, and each sensor requires 2 AA alkaline or lithium batteries (not included). Lithium batteries are recommended for use in sensors for temperatures below 4 degrees Fahrenheit (-20 degrees Celsius). By using this combination of temperature gauge and freezer monitor, you will rest assured that your family's food is stored safely and appropriately. This digital refrigerator and freezer thermometer Comes with a one-year limited warranty.
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Uses 433Mhz frequency to communicate, in case you are planning to integrate it with Home Assistant or other smart house systems, will require a 433MHz gateway or SDR (software-defined radio)

https://www.amazon.com/gp/aw/d/B004QJVU78