Restoring viber only from icloud

For some third-party apps, you may need to download data from the app provider again.

About your apps, music, movies, and TV shows

iCloud Backup includes information about your purchase history from the Apple Music app, Apple TV app, Apple Books app, Apple Podcasts app, and the App Store. After you restore from iCloud Backup, your purchased media automatically downloads again from Apple services. Availability of redownloads varies by country or region. Previous purchases may be unavailable when they've been refunded or aren't available in the store anymore. Some previously purchased movies may not be available for redownload from Apple TV.

Viber should be able to help you. They have recorded everything you have ever done on their service, in addition to taking a copy of every detail of each of your Contacts. So they should be able to give you a copy of the bits of your life that you want to recover.