This is no standalone app. You need to install IP Widget (https://play.google.com/store/apps/details?id=de.dieterthiess.ipwidget)
Get sms notifications on external IP address changes
Get information about external IP address changes by text message.
You need to enter the recipient phone number and grant the permission to send sms.
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.
Warning: This plugin can cause costs if you do not have a flat rate for text messages!