I have sort of solved this by having this in the .htaccess file:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST} ^www\.sysctl\.dk$ [OR]
RewriteCond %{HTTP_HOST} ^sysctl\.dk$
RewriteRule ^(.*)$ https://interstellar.dk/$1 [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . index.php [L]
</IfModule>
That permanently redirects all old URLs using sysctl.dk/blog/ to interstellar.dk