MEATER Block, MEATER Pro Duo, and MEATER Pro XL need to be on the same WiFi network as your phone or tablet for the MEATER app to monitor your cooks.
If you notice that your MEATER device connects to WiFi but can't communicate with the app, it's most likely because your MEATER device and mobile device are on different networks, or your phone or tablet isn't connected to WiFi at all.