WhatsApp Wizards Needed. How to make WhatsApp recognise backup from old number

I almost looked into every single forum out there. Anyone who REALLY knows an answer that REALLY works (please don't copy stuff from WhatsApp forum, already tried everything there)?

Here's what happened.

I had a Whatsapp account with an old number. My old number expired but WhatsApp still worked.

I bought a new number, but still used WhatsApp on my old number.

I uninstalled WhatsApp.

I reinstalled WhatsApp, introduced my new number, and whatsapp did not recognise my local backup.

I have no Google Drive backup.

So now all my old conversations and groups are gone, some of which are really important. How do I make WhatsApp recognise my old number backup?

I'd be grateful if anyone can help!