Point homepage of onlineshop to different server then rest of the shop

I have an onlineshop running on Server A. Now I created a landingpage with WordPress on Server B. My problem: I want the homepage (index page) of the shop to be the landingpage from Server B. How can I do that in the best way?

Example how it should be: www.shop.com (homepage, index, from server B) www.shop.com/product1 (normal content from server A)

I thought about iFrames, but they are not good for SEO and also it doesn't look that good. I thought about just DNS changes, but that's obviously not possible. I am really lost at the moment.