Let's go to builtwith.com - this site is something like a knowledge base about technologies.
If you enter the site you need into the search bar of the site, you will receive complete information about it.
In the E-commerce column you will see the name of the merchant (payment gateway) that is used on the site.
If this method does not work, try writing to the site’s technical support and asking them. Explain this by saying that your card is not supported by all payment gateways and it is important for you to know which one is being used, or come up with another reason.
Also try opening the payment page in html mode and looking at the name of the merchant (payment gateway) in the line of code.
Each payment gateway has its own characteristics of working with cards. Try looking for topics on the forum that tell you which cards or which method is suitable for successful driving.