If you have unread messages on Viber, then the only way is to install the application again and use the same mobile number to log-in. As far as I know, the messages are stored for a limited period of time before they're actually deleted, even for the unread messages. Therefore, install the app and use the same mobile number to see all the unread messages. You could also try installing the Windows application, but that implies you didn't use the Deactivate account option.