This app requires a device in your local network which runs 24/7 (like a home-server or a pi). Install the following script on the device (https://github.com/Jako9/WOL_Server) by following the steps provided on the website.

Once this is done, you can specify all relevant information upon starting the app for the first time.

You can now add devices via their MAC-Addresses.
You can also add devices outside of the specified local network by configuring custom IP, Ports and Keys for them.