Cannot add jQuery to Rails 6 projects

I've been trying to make a few apps work with Fomantic UI and the gem works fine, the problem is that to use calendars and modals you need jquery... No probs, I added that by following this guide and its still not working. On deployment, it will just not load jQuery at all. I've been forced to include cnd links on the application.html.erb to load jQuery, there must be a way of doing this. (?)