Turn off your wifi in settings on the phone so you're phone won't actively search for wifi thus saving battery in the meantime.
If
Disconnects from any WiFi network
This Trigger fires every time your Android device disconnects from any WiFi network.
Then
Send a notification from the IFTTT app
This action will send a notification to your devices from the IFTTT app.
Explore more great ways to automate Notifications and Android Device