Restore WhatsApp History from old Number
Afaik there is no other possibility restoring WhatsApp messages than doing it via an iPhone backup. WhatsApp officially does not store messages except those, which can't be directly delivered. WhatsApp will always refuse giving you access to this kind of data.

So your only chance will be to find an old backup of your device and try to back your device up with that old state.

Please also provide some information, if your issue is solved or what other step you took to figure it out.

Edit to be clear: WhatsApp history backups are only stored in your device's backup :)