Hello!!
I was trying to navigate through the courses created on my site via an iPhone and there I came across the issue where the button for the next unit at the end of the unit is not accessible on IOS devices. Irrespective of the browser I tried I had the same issue all over them. Whereas the same units under the same courses when opened on Android devices are very user-friendly.
For your reference, I'm adding some screenshots displaying the problem over the IOS device (mainly high-end) and Android devices I used to test.
And the below image indicates the desktop version of the same unit under the same course.
*Note: This is not a problem over a single unit or course this is a global problem faced while using iPhone.
I'd request you to try using any of the courses on an iPhone, not a simulator but a real iPhone cause only then the browser bars will be a barrier to the next unit button.