There have been lots of complaints about the Wi-Fi in these forums, and I've never been able to get mine to work reliably. The issue seems to be that the reception is very poor, so if you don't have a strong signal in your kitchen it just drops the connection or won't connect at all. And the bluetooth feature is pretty useless, IMHO. If you're close enough for a bluetooth connection (generally under 30 ft), what's the point of using the app vs just setting it manually? Plus, I've had experiences with the unit resetting/mis-setting when bluetooth is lost and reestablished, which creates serious issues for cooking.
That said, I've never had any issues when setting the device manually, so that's what I always do. I'm not really happy about having paid for features that don't work correctly, but the core heating function is solid.