Notifications will help your users to understand where they are in their verification process. A notification system will highly help conversion rate of verified user.
As Synaps does not collect email addresses of your users, you should setup a notification system by mail.
Usually, it happens when you receive a webhook. If the user verification is rejected, you will have the reason of rejection in the webhook. You should then send an email to your user to ask them to go to your verification page. They will be able to re-submit their documents. If the user verification is valid, you should send then an email to inform them that they are verified.
Check out webhook integration