You have a couple of errors in the console relating to JQuery, Bootstrap and Modernizr which might need addressing for starters
The bootstrap error suggests that you might need to use an older version of JQuery, pre version 3. Whether doing so will cause issues elsewhere I do not know.
This might not fix the specific problem you are having, but as I say it is a start.
Secondly have you looked into options for debugging on the iphone? Not something I have done on an iphone, but this link might be worth looking into.