Hi thindpls,
first thing first; HD Quiz is 100% compatible with iPhones.
HD Quiz also has nothing to do with logins or anything like that. My best guess is that this is a caching issue either with your site or the iPhone you are using.
I’m also unable to replicate the issue on my own phone on your site.
I see that you are using WP-Optimize, so the first thing to do is go to that plugin’s settings and clear/rebuild the cache. This will ensure that any other errors you may have are flushed out allowing your site cache to be rebuilt.
The second and most important step is to clear your phone’s cache as well. Apple products and especially iPhones are notorious for aggressively caching pages. They do this in order to always feel fast since you don’t need to redownload assets that are already cached. The problem with this is that if one of your assets is corrupt, then this corrupt asset will also be cached.
On your phone,
Step 1: Tap to open Settings
Step 2: Scroll down and tap Safari. It’s listed with Apple’s other first-party apps.
Step 3: Tap Clear History and Website Data.
Step 4: Tap Clear History and Data.
Continue through the steps, but make sure that ONLY website data is selected to be cleared. Otherwise, it will also clear your browsing history etc.
Please let me know once you’ve done the above and I will continue to assist you and help debug if clearing the cache does not fix your site.