Here are some links that may help you
If you change your device frequently or changed your Phone number too frequently then this is against the WhatsApp policy terms and conditions, and you are bound to land in trouble.

The process which I use...
Log into WhatsApp with the same Phone number that was used to backup. If it prompts you to update your chat history, then you use it or else check the Phone Number. WhatsApp chat history can be deleted via WhatsApp only.
Even if you erase your iPhone the chat history remains on iCloud Drive and is accessible only through WhatsApp.
WhatsApp Backup is taken on iCloud and while restoring, it verifies your Phone number.
- The phone number used for backing up and restoring must be the same. You can't restore the chat history from another WhatsApp account.
- Because backups are tied to individual phone numbers, it is possible to have backups for multiple WhatsApp accounts stored in the same iCloud account.