Access custom block in page.html.twig template
Take away the block from the view header and use twig_tweak module to be able to insert something like this in your twig template

{# No need to configure this block on "admin/structure/block" page. #}
&lt;dt&gt;Block:&lt;/dt&gt;
&lt;dd&gt;{{ drupal_block('BLOCKID') }}&lt;/dd&gt;


Or if you don't want to use an extra module add to your THEMENAME.theme

function THEMENAME_preprocess_page(&amp;$variables) {
  $block = \Drupal\block\Entity\Block::load('BLOCKID');
  $content = \Drupal::entityTypeManager()-&gt;getViewBuilder('block')-&gt;view($block);
  $variables['my_block'] = $content;
}


and in your twig template just use

{{ my_block }}