Do you have anonymous access setup in your application, and have you assigned the Deeplink.User role to your anonymous user role?
From the documentation:
Make sure that all roles—including your guest roles—which need to access your app via a deep link have the DeepLink.User user role. Otherwise, they will not be able to use any link, because access to objects owned by the Deep Link module will be denied.
https://docs.mendix.com/appstore/modules/deep-link/#33-configuring-security-settings
Good luck!
The deep link must be configured to provide access to the desired page without having to log in to the application, otherwise it causes a 404 error.