Checkboxes on iOS (iPhone/iPad) not working

It's my first post here and I have to admit - I'm not a developer of any sorts.

I'm using Brolmo's "Time Slots Booking Calendar" for a laundry booking system.

It's running a JavaScript and a stylesheet: http://www.fromfrom.dk/v.html

The issue: iOS-users can't click on any of the checkboxes (tested on iPhone 8, 9, X) running the newest version of Safari and Chrome. Unfortunately, I do not own an iOS device to test this myself. The checkboxes works great on Android running Chrome, as well as Windows running IE, Edge, Chrome and Firefox.

I've tried contacting Brolmo, but they keep insisting that it runs perfectly on iOS. Therefore I would really appreciate a little help :-)

If any of you need more information, please let me know.

Thanks in advance