All modern mobile operating systems centrally select the most power efficient method of determining the phone’s position from the different location providers available (GPS, Wi-Fi, cell towers). With Wi-Fi as the location provider your phone can locate its position simply based on the set of Wi-Fi networks that can be detected in the area where you are, so keeping the Wi-Fi sensor on, even when you are currently not connected to a Wi-Fi network, is extremely important. In this way, your smartphone learns your frequent locations based on the networks detected and notifies apps like tado° when there is a location change.
Using the Wi-Fi reduces the GPS sensor activity and battery consumption considerably while delivering equally reliable or even better position measurements than GPS alone. This is the reason why Apple iOS strongly suggests that you keep the Wi-Fi always activated and sends notifications when Wi-Fi is deactivated.
If your phone runs Android then tado°'s Geofencing should recognize that you are home when you connect to your home Wi-Fi network, so make sure that the Wi-Fi-scanning is enabled.