Which cloneable project did you use when starting the project? It may help try to figure out what’s different on your version, or at the very least, give you a point of contact to the original creator in the event you need to get more assistance.
Since custom code isn’t triggered in the Designer, even the linked template won’t work until you view the published project. Just keep in mind that you should keep the class names the same on these slider arrows as that’s how the code is able to find the correct element on the page.
Give it a try once you publish on your staging domain and I’d bet it will work just fine