This definitely sounds odd, but, like you I was able to visit the page on multiple mobiles in Safari.
You have flexbox on all your pages. That might not be it.
Can you please provide some more details so we can see why the issue is occurring?
What do they mean by not working? It doesn’t load at all? What error message do they get? Can they send a screenshot of what they see, please?
Can they visit the products page?
I see that you have a min-height of 0% on the body of the summit page and the products page.
Some safari versions do not like percentage values so that might be a cause. Let them test if they can see the products page. You can also try to remove the min-height value on the body and ask them to test again.
Any information that you can provide, can help us resolve the issue faster.