Documentation suggest that you need to register the custom model with admin and also define few methods as well so that admin interface works with custom user model.
You may also have to define built-in forms for User.
From Custom users and django.contrib.admin
You will also need to register your custom User model with the admin. If your custom User model extends AbstractUser, you can use Django's existing UserAdmin class. However, if your User model extends AbstractBaseUser, you'll need to define a custom ModelAdmin class.