Can you try this at search.php
<?php if ( have_posts() && strlen( trim( get_search_query() ) ) != 0 ) : while ( have_posts() ) : the_post(); // Start of the loop.?>
<?php // Do code for the loop. ?>
<?php endwhile; // End of the loop. ?>
<div class="navigation">
<?php
if( function_exists( 'wp_pagenavi' ) ) {
wp_pagenavi();
}
?>
</div>
<?php else:?>
<b>No results match your search.</b>
<p>For Accurate results, Please Search the Model Number of the device or the Brand Name.</p>
<?php
endif;
?>