Is your landing page a custom HTML page or a page created in mx studio?
If it's a custom page you should redirect to this page from index.html (use meta tag). You can find this file in the theme folder of your project.
If the landing page is a page created within studio, you should enable anonymous access in your project runtime settings and use this page as default in your navigation settings. Set anonymous to have access to this page.
Have a look here to read more about anonymous access: https://docs.mendix.com/refguide/anonymous-users