From the Meater FAQ:
Aren't BBQ grills, smokers, and ovens essentially "Faraday Cages" that prevent RF signal transmission?
In theory, a Faraday Cage will block RF signals from entering and exiting the cage. The good news is that BBQ grills, smokers, and ovens are not built or designed to be Faraday Cages.
A good Faraday Cage has no gaps, slots, and holes; Are usually made of thick conductive material and designed with as little individual parts/pieces as possible (i.e. every joining edge or corner between materials are spot welded, etc.).
In reality BBQ grills, smokers, and ovens are anything but a Faraday Cage. They have plenty of large air vents and gaps. If air and smoke can enter or exit your BBQ grill, smoker, and oven, then it’ll be easy for 2.4GHz radio frequency to enter and exit through the same gaps and holes.
One way to test is to place a Bluetooth enabled device (such as a speaker) inside your grill and test its connection with your smartphone.