"Back to top" works on desktop, tablet, but not mobile

On Derby Home Rental, I added html for a “Back to top of page” link to appear at the bottom of each page. The link works fine, redirecting visitors to the top of the page, on my browsers on my MacBook Pro and my iPad. It does not, however, work on my iPhone 6 running iOS 9.2.1 using Safari or Firefox 2.1.

Why doesn’t “Back to top of page” work on my iPhone and—more importantly—how can I fix it? Thanks!