How can I add a pager to the top of a forum?
Pager is known within the "views-view.html.twig" file: /core/modules/views/templates/views-view.html.twig

If you are using a contributed or custom theme you can override that file and move the pager output where you'd like. So your new file would live here:
/themes/custom/MY_THEME/templates/views-view.html.twig"
 for example.

As near as I can tell there's no setting for this in core currently, although it's been requested.

See also: https://api.drupal.org/api/drupal/core%21modules%21views%21templates%21views-view.html.twig/8.7.x

https://api.drupal.org/api/drupal/core%21modules%21views%21views.theme.inc/function/template_preprocess_views_view/8.7.x