Serif’s software purchases (like most other software companies) are dependent on the email address used when first buying it.

To check, I’ve just had a quick search through my old emails using the search term ‘serif order’ and found the ones I received when I first bought my copies of Affinity Designer and Photo. If you haven’t done this, it might help to track down your original order confirmation emails. If not, maybe you could find the transaction on a bank statement. Good luck anyway.