how to transfer viber data from icloud to new iphone

Normally Viber stores their data on their Cloud Services and not on iCloud.

Downloading and installing Viber on your new device and logging with your Viber account should be good enough to get all the Viber chats and messages

Viber Messenger: Chats & Calls is not an Apple product. Contact the app developer here --> App Support or Viber Support

Only Viber data may not be possible. You need to restore your device with iCloud Backup to restore Viber and other app data.