Can't scroll horizontal in Deck with my Ipad Pro

Hey Everybody,
I am hoping that someone can solve my actual problem. I installed Nextcloud on a Server at my University to collaborate with my Boss on Homeworks which come from our students.
So we’re organizing the students in Deck in lists, where they travers across all sections, like

  • Signed in for course
  • submitted first homework
  • waiting for correction of first homework
  • and so on…
  • passed

This system works like a charm for ME. But my Boss has the following Problem:
He works on an Ipad Pro in Safari, but there, it is not practicable possible to scroll horizontal above our Deck List. Is there any option in Safari or Nextcloud where we can make this scrollbar thicker or something? We have ca 10 lists, so working in Landscape mode will not solve the problem. If he scrolls via touch with his fingers, he moves the cards in the list to another list.
Any suggestions are welcome, in the actual status I’m afraid we have to go back to Email and Directories on our Harddrives.