This is no standalone app. You need to install IP Widget (https://play.google.com/store/apps/details?id=de.dieterthiess.ipwidget)

Calls an URL when the external IP address changes.

You can configure an url with placeholders {ip} for the external ip address and {network} for wifi name.
After entering the data, you can save these settings and the setup of the plugin is completed.

If you want to monitor your external IP address changes while your device is connected to the same wifi all the time, you need to enable the manual update in IP Widget settings, so the device will update data periodically and will detect changes of the external ip address.