New users do not receive mails and can not log in

  1. You should not need to change the realm settings in your cloud instance. The defaults are fine you can't and there is no need to set the native to order 0.

  2. New users do not receive mails

    They shouldn't , this is not a functionality that elastic cloud offers at this time ( i.e. deployment users getting notified of account creation via email )

  3. I have been researching a bit and found this thread.

    This is about and on premise installation and not applicable to your case

Can you clarify where do you try to login ? Is it the same kibana instance as the one you created the user in? What version is your kibana + es cluster ?