Recover whatsapp chat backups after reset without access to old number
The WhatsApp backups are encrypted with a key which is connected to your account. This ensures that the backup only gets restored if the user connects the same phone number. If not, this backup holds zero value for WhatsApp. 

This also means that the database backup (on your SD) card cannot be read. So, you shall not be able to decrypt and bring it to a readable version. I'll really look for possibilities of getting the old number re-issued or something. 

Without access to that old number, there's no chance (as per my knowledge) to decrypt the database and restore it or make it readable.