Hey Lea,
It sounds like you're trying to use a custom controller to serve content while using content tree-based routing in Kentico Xperience 13?
Have you added the RegisterPageRoute assembly attribute on the controller? For example, adding the following above the controller/namespace?
[assembly: RegisterPageRoute(Home.CLASS_NAME, typeof(HomeController))]
If you're not using content tree-based routing, then ensure the right setting for Routing mode is set in the Settings application, in the URLs and SEO category under the Routing section. Then you need to ensure you are setting a URL pattern in the Home page type to /
I'd recommend reading over this page in the documentation.