Since users won’t be authenticated when they open the app for the first time, they are assigned the anonymous user role. You should set the sign-in page as the homepage for this role only.
After sign-in a user will be authenticated and receive a different user role. They will be taken to either the default home page (so make sure the sign-in page is not your default) or a role-specific homepage of your choosing. In the navigation menu you can set a homepage per userrole.