Allow the role “Anonymous” to access the login page (when you create a custom login page in Mendix). Each page that is accessible to the user (either signed in or anonymous) require at least one role
Open the page NewLoginPage, in the Properties Pane: Navigation: visible for: add Anonymous
Review the learning path: https://academy.mendix.com/link/modules/183/lectures/1419/3.4-Custom-Login-Page