You must be connected to either wifi or cellular data to use the Hailo app. If you move to an area of your restaurant that's a deadzone (eg your cooler), you will be temporarily disconnected.
You'll get an alert on your screen if this happens, and you won't lose your order so far. When you reconnect, your session will resume and you can complete your order.
If your store has unreliable wifi, it would be best to turn your wifi off and connect using cellular data only, to avoid any connection issues.