It works for me (with te theme styling). Add it in functions.php

/*************************************/
add_filter( 'avf_template_builder_content', 'enfold_customization_portfolio_nextprev' );
 
  function enfold_customization_portfolio_nextprev( $content )
  {
  	global $wp_version;
	        $settings = array();
	        $settings['same_category'] = 'true';
	        $settings['excluded_terms'] = '';
			$settings['wpversion'] = $wp_version;
			$settings['type'] = get_post_type();
			$settings['taxonomy'] = ($settings['type'] == 'portfolio') ? 'portfolio_entries' : $taxonomy;

			 if(version_compare($settings['wpversion'], '3.8', '>=' ))
	        {
	            $entries['prev'] = get_previous_post($settings['same_category'], $settings['excluded_terms'], $settings['taxonomy']);
	            $entries['next'] = get_next_post($settings['same_category'], $settings['excluded_terms'], $settings['taxonomy']);
	        }
	        else
	        {
	            $entries['prev'] = get_previous_post($settings['same_category']);
	            $entries['next'] = get_next_post($settings['same_category']);
	        }

	        $entries = apply_filters('avia_post_nav_entries', $entries, $settings);
        	$output = "";

  	if ( is_singular( 'portfolio' ) )
    {
    		$entries = apply_filters('avia_post_nav_entries', $entries, $settings);
        	

        	foreach ($entries as $key => $entry)
		{
            if(empty($entry)) continue;
			$the_title 	= isset($entry->av_custom_title) ? $entry->av_custom_title : avia_backend_truncate(get_the_title($entry->ID),75," ");
			$link 		= isset($entry->av_custom_link)  ? $entry->av_custom_link  : get_permalink($entry->ID);
			$image 		= isset($entry->av_custom_image) ? $entry->av_custom_image : get_the_post_thumbnail($entry->ID, 'thumbnail');
			
            $tc1   = $tc2 = "";
            $class = $image ? "with-image" : "without-image";

            $content .= "<a class='avia-post-nav avia-post-{$key} {$class}' href='{$link}' >";
		    $content .= "    <span class='label iconfont' ".av_icon_string($key)."></span>";
		    $content .= "    <span class='entry-info-wrap'>";
		    $content .= "        <span class='entry-info'>";
		    $tc1     = "            <span class='entry-title'>{$the_title}</span>";
if($image)  $tc2     = "            <span class='entry-image'>{$image}</span>";
            $content .= $key == 'prev' ?  $tc1.$tc2 : $tc2.$tc1;
            $content .= "        </span>";
            $content .= "    </span>";
		    $content .= "</a>";
		}

  	return $content;
  	}
    else {
  	return $content;
    }
  }