Template suggestion twig.

template_suggestion.php

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function ixia_theme_suggestions_eck_entity_alter(array &$suggestions, array $vars, $hook) {
  $entity = $vars['entity']['#slice'];
  
  if(in_array('eck_entity__slice__resources_slice', $suggestions)) {
    $template_suggestion_array = $entity->field_template_suggestion->getValue();
    $template_lenght = count($template_suggestion_array);
    if($template_lenght) {
      $suggested_template = $template_suggestion_array[0]['value'];
      $suggestion = $hook . '__ts__' .$suggested_template;
      $suggestions []= $suggestion;
    }
  }
}