IOS doesn’t support background-attachment fixed but will generally scroll with the element as usual. However when you add background-size cover as well it seems to break the whole thing. We had a similar question here.
You can use a media query on device width and detect ipad and iphone and offer background-attachment scroll instead.
If it were only one fixed background you needed you could use a position:fixed div instead with a normal background image but that approach won’t work for multiple images.
I am away this week and can’t test but I note my old example with cover works on the iphone but scrolls instead of being fixed. I think it may work because the div has a set height. I need to test more when I get back.