Check out the YoLink temperature sensors on Amazon. They are amazing for monitoring our secondary freezer. All their sensors have been great. Temp for fridge/freezers, leak detectors, temp+humidity for basement, they have a motion sensor that is a magnet you can place in the back of your mailbox if you want to know when mail is delivered. Their door sensor can be installed on overhead garage doors by a magnet and a velcro strip for the part that goes on the wall. They also have a 1/4 mile range and the sensor can be inside your fridge/freezer and still connect. App is great, no monthly fee, you get notifications on your phone you can also get a small number of SMS messages for free. All notifications/limits are configurable.