The primary call to Action on your product page should be the “Buy button”. Make it a large sized button in a contrasting color to the background that reads ‘Buy Now’ or ‘Add to Cart’ or ‘Add to Bag’ etc. In short, distinguish it from other elements of the product details page. Make sure it comes above all your secondary CTAs and that there is enough space around your add to cart button so that it is easy to spot and doesn’t feel cluttered.
You can also include visual cues in your add to cart button such as a shopping cart icon. And remember, always A/B test your primary CTA.