Retrieving old WhatsApp chat history from iCloud

How you retrieve what you have backed up is to delete the app (NB theteby deleting ALL current chats) then reinstall the app and restoring the Whatsapp backup.

NOW when did you last backup? ie in the settings inside Whatsapp what backup settings are set. Chats are not backed up in your regular phone backup,

WhatsApp requires specific steps to be taken to save and restore your Chat History in iCloud. In some cases, they won't allow it at all as it is encrypted.

https://faq.whatsapp.com/618575946635920/?cms_platform=iphone&helpref=platform_switcher