Here are some related articles you can follow. They provides many recovery and backup ways for Android users. You can choose one you fancy.
Easy Ways to Backup & Restore Viber Messages On Android
How to Retrieve Deleted Viber Messages/Chat History on Android Mac
If you have back your viber messages up somewhere like Google account, you can directly recover from them. If not, you may need a data recovery tool.
Android Phone Recovery is a good choice. It offers users a simple and safe way to recover lost Viber messages, contacts, text messages, photos, videos, audio, WhatsApp messages, etc from Android phone internal memory.
As a specific Android data recovery software, this program can recover deleted files for most popular Android brands including Samsung, HTC, LG, Sony, Motorola, ZET, Huawei, etc. Also, it is compatible with all versions of Android OS, including the latest Android 6.0 Marshmallow. Hot devices like Samsung Galaxy S6, S5, S4, Note5, Note4, Note3, HTC One M9, M8, LG G4, G3, Sony Xperia Z5, Z4, Motorola Droid RAZR, etc are all well supported.
Related Videos:
Easy Ways to Retrieve Viber Messages on Android https://youtu.be/2w6dF1TZJOY
How to retrieve deleted viber messages/ chat history on Samsung https://youtu.be/A2DxgsZqeB8