You just have to tell the app which one is your home wifi and keep your router on. Ismyfridgeon will detect when you are outside home and will notify you when it cannot reach it.
If you have a stable internet connection and keep the router usually on it should work quite well. Ismyfridgeone tries a few times before reaching to a conclusion, and has into account the quality of the current internet access (for example, the app won't show any notification if it cannot reach home while you are in a garage with a bad connection).
This app does not send data to any server and the data consumption is very low (<200KB in a month).