How to add AppleCare+ within 60 days of purchase?

Hi!

Recently I bought AirPods Pro 2 from a seller that was brand-new and sealed.

I was told by them that I was able to buy AppleCare+ within 60 days of purchase, but even after a few weeks I still couldn't buy it, and kept getting the same message every time, "It looks like that device isn't eligible for AppleCare+.".

Recently, I called Apple Support and they have told me that they can not add AppleCare+ since I bought it from a 3rd party seller, but I thought I can add it 60 days of whenever I bought the product? Regardless of where I bought it?

I have the receipt of when I got it, and I know that this product is 100% genuine.

Please let me know if there's any way of adding AppleCare+ to this product, as the 60-day time period is almost up.

Thanks!