When you purchase TechEDMentor, but still cannot use the advanced features, just restore it.
If it shows “Failed to restore previous purchases”, please make sure:
1. You use the same Google Account previously used to purchase the app.
2. You've made a valid purchase.
3. Google Service and Google Pay are available in current region.
If you still have questions, please drop a line at support@techedmentor.com.
Support Chat 
TechEDMentor