Best product to brighten a white Dress

I would try Ace laundry bleach, and dry outside in the sunshine.

Vanish Oxi Action Crystal White or similar.

Vanish or the Lidl equivalent work well. Chinese shops sell a laundry blueing powder which brightens whites.

I’ve use Glo white sachets before.

And then, when one of these methods has brightened it, be sure to wash it only with white laundry, to make sure it stays bright white.