Single Bluetooth monitor of either the fridge or freezer.

This single unit is great for monitoring the internal temperatures of the fridge or the freezer. Only requires Bluetooth so works without wifi . Simply download the app and it connects to your phone. Needs two AAA batteries to run and once installed will last months. 
The instructions include all details required to download the free app and set the unit up.

You can set alerts for the fridge or freezer if it falls below a set temperature.

Will keep historical data so you can see if the fridge climbed too high while traveling. 

This unit is something you will come to depend on.