Adding Createt Custom Field inside page-title.html.twig not working.

Hello everybody,

I am new to Drupal and kind of a newbie.
I used the Blog Module https://www.drupal.org/project/blog to create my blog.
Now i wanted to add a custom field that i created to the Page Title on the Blog Post (Full View of Blog Post).
If i add the custom field in the Blog Page (all blog posts view) it works fine.

File (node--view--blog--blog--all.html.twig):

{%
  set classes = [
    'node',
    'node--type-' ~ node.bundle|clean_class,
    node.isPromoted() ? 'node--promoted',
    node.isSticky() ? 'node--sticky',
    not node.isPublished() ? 'node--unpublished',
    view_mode ? 'node--view-mode-' ~ view_mode|clean_class,
  ]
%}
{{ attach_library('classy/node') }}
<article{{ attributes.addClass(classes) }}>

  {% if display_submitted %}
    <footer class="node__meta">
      {{ author_picture }}
      <div{{ author_attributes.addClass('node__submitted') }}>
        {{ metadata }}
      </div>
    </footer>
  {% endif %}

  <div{{ content_attributes.addClass('node__content') }}>

    {{ content.field_blog_image }}

    <div class="date-blog">
      {% set createdDate = node.getCreatedTime|format_date('blog_date') %}
      {% trans %} {{ createdDate }} {% endtrans %}
    </div>

    {{ title_prefix }}
    {% if not page %}
      <h2{{ title_attributes }}>
        <a href="{{ url }}" rel="bookmark">{{ label }}
        {{ content.field_untertitel }}<!-- THIS IS MY CUSTOM FIELD THAT I ADDED AND ITS WORKING -->
       </a>
      </h2>
    {% endif %}
    {{ title_suffix }}

    {{ content.body }}
    
    {{ content.links }}
    
  </div>

</article>

When i add it like this in my page-title.html.twig File it doesnt work at all.

File (page-title.html.twig):

{{ title_prefix }}
{% if title %}
  <h1{{ title_attributes.addClass('page-title') }}>{{ title }}</h1>
{% endif %}
{{ title_suffix }}

<!-- This Part is added by me but doesnt work -->
{% if not page %}
	<span class="sub-title">
		{{ content.field_untertitel }}
	</span>
{% endif %}

What am i doing wrong here?
Thank you guys for helping <3