Unfortunately, you are not alone in facing this problem.
The best solution I found is to ask a family member or friend in an authorized country to connect on a PC to Google Play with the gmail account you used to register your Android device and ask them to purchase the app. This one will then be automatically installed on your Android when your device will be connected to the Internet.
Here is the link to the app on the Play store: https://play.google.com/store/apps/details?id=com.sailgrib_wr.paid
And the list of authorized countries: https://support.google.com/googleplay/answer/143779?hl=en
You can possibly perform this procedure by taking control of the PC remotely using a service like TeamViewer.
Otherwise, you can buy the application from a PC via a proxy to make Google believe you are an authorized country.