Editing robots.txt

Hi,

So I am trying to add stuff to the robots.txt file and am not sure if it goes through.

I go to Rank Math -> General Settings -> Edit robots.txt and add the stuff I want to add. I hit Save changes and get the message “Settings changed.”

I look at the robots.txt file (it is not stored on the server but it’s dynamic) and the changes have not been applied.
So I have two questions.

1) How long does it take before the changes are actually applied?
2) Shall I add the default text from the sitemap in the editing box or will that not be removed anyway? I am talking about these lines:

User-agent: *
Disallow: /wp-admin/
Allow: /wp-admin/admin-ajax.php

Sitemap: https://site.com/sitemap_index.xml

Hope you can help this is really important. Thanks!