https://wpvkp.com/enable-dns-prefetch-wordpress-instant-articles-plugin -- This code will help you to enable dns prefetch in wordpress.

functions.php

<?php // ************* DON'T COPY ME ***************

/* This is for homepage */
function wpvkp_dnsprefetch_homepage() {
  $homepage = get_home_url();
  if ( !empty( $homepage ) ) {
    echo '<link rel="prefetch" href="'.$homepage.'">
      <link rel="prerender" href="'.$homepage.'">';
  }
}
add_action( 'wp_head', 'wpvkp_dnsprefetch_homepage', 10 );