(Drupal 8) A simple back/next pager (based on a views result) for node default display mode.

mytheme.theme

<?php

use Drupal\views\Views;

// Optional, if using filter values passed via URL (see example below).
use Drupal\Component\Utility\Html;

/**
 * Preprocesses variables for node--NODE_TYPE.html.twig.
 *
 * Replace 'mytheme' with the name of your theme, and NODE_TYPE and VIEW_NAME
 * with actual machine names.
 */
function mytheme_preprocess_node__NODE_TYPE(&$variables) {

  $node = $variables['node'];

  // Execute the view that provides the context.
  $view = \Drupal\views\Views::getView('VIEW_NAME');

  // Optionally set a contextual filter value passed in via the URL;
  // e.g. path-to-node?vid=456. You will need to add the parameter to the pager
  // URLs in your Twig template.
  $variables['arg'] = Html::escape(\Drupal::request()->query->get('vid'));
  $view->setArguments([$variables['arg']]);

  $view->execute();

  // Iterate through the results to find the results index for previous,
  // current, and next nodes.
  $match = FALSE;
  foreach ($view->result as $k => $row) {
    // Break out of iterator when we find the next row.
    if ($match !== FALSE) {
      $next = $k;
      break;
    }
    // This result matches our currently loaded node.
    if ($row->nid == $node->id()) {
      $match = $k;
    }
    // As long as we haven't reached a match, let's keep updating $previous.
    if ($match === FALSE) {
      $previous = $k;
    }
  }

  // If a match is found, set up our template variables.
  if ($match !== FALSE) {
    $variables['node_view_pager'] = [];
    $variables['node_view_pager']['num_results'] = count($view->result);
    $variables['node_view_pager']['index'] = $match + 1;
    $controller = \Drupal::entityManager()->getStorage('node');
    if (isset($previous)) {
      $variables['node_view_pager']['previous'] = $controller->load($view->result[$previous]->nid);
    }
    if (isset($next)) {
      $variables['node_view_pager']['next'] = $controller->load($view->result[$next]->nid);
    }
  }
}

node--NODE_TYPE.html.twig

{#
/**
 * @file node--NODE_TYPE.html.twig
 *
 * Custom variables:
 * - node_view_pager: An array containing the pager information.
 *   - node_view_pager.previous: The previous node object.
 *   - node_view_pager.next: The next node object.
 *   - node_view_pager.index: The index of the view result (incremented by 1 so
 *     it starts at '1' instead of '0').
 *   - node_view_pager.num_results: The total number of results returned.
 */
#}
<article{{ attributes }}>

  {{ title_prefix }}
  {% if not page %}
    <h2{{ title_attributes }}>
      <a href="{{ url }}" rel="bookmark">{{ label }}</a>
    </h2>
  {% endif %}
  {{ title_suffix }}

  {% if display_submitted %}
    <footer>
      {{ author_picture }}
      <div{{ author_attributes }}>
        {% trans %}Submitted by {{ author_name }} on {{ date }}{% endtrans %}
        {{ metadata }}
      </div>
    </footer>
  {% endif %}

  <div{{ content_attributes }}>
    {{ content }}
  </div>

  {# Begin node view pager #}
  {% if node_view_pager %}
  <nav class="node-pager">
    <ul role="navigation" aria-label="Pagination">
      <li>
      {% if node_view_pager %}
        {% if node_view_pager.previous %}
          <a href="{{ path('entity.node.canonical', {'node': node_view_pager.previous.id}) }}"><span>{{ 'Back'|t }}</span></a>
        {% else %}
          <span>{{ 'Back'|t }}</span>
        {% endif %}
      {% endif %}
      </li>
      <li>
        {% trans %}
          ({{ node_view_pager.index }}/{{ node_view_pager.num_results }})
        {% endtrans %}
      </li>
      <li>
      {% if node_view_pager %}
        {% if node_view_pager.next %}
          <a href="{{ path('entity.node.canonical', {'node': node_view_pager.next.id}) }}"><span>{{ 'Next'|t }}</span></a>
        {% else %}
          <span>{{ 'Next'|t }}</span>
        {% endif %}
      {% endif %}
      </li>
    </ul>
  </nav>
  {% endif %}
  {# End node view pager #}

</article>