Hello,
Unfortunately, we don’t have an ability to test and debug your website with iPhone 6s.
But if other sites with our theme are working correctly then it seems that it is not Basel problem. Anyway here are some steps that you can perform to detect what is it caused by
1. Disable all plugins one-by-one and check every time.
2. Remove all additional customization (CSS & JS codes).
3. Create some blank page (without any Visual Composer elements) and check if it caused by some element.
Hope this will help.
Kind Regards