Shortcode not working & the relation to the_content()

[codeispoetry]

My short code was not working on my custom theme, but the same short code, when used in twenty fifteen, was working.

I researched over google and found that it was something related to not using the_content correctly.

1. https://kb.yoast.com/kb/shortcodes-not-rendering/
2. https://wordpress.org/support/topic/shortcodes-not-working-custom-theme/

I applied the solution suggested in the Yoast →

that means I changed from

<?php the_content(); ?>

TO

<?php echo apply_filters( 'the_content', $post->post_content ); ?>

But still, the shortcodes are not working.

what is the working fix?

I have this code in single.php →

<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
                            <?php $post_id = get_the_ID(); ?>
                            <?php get_template_part('content',get_post_format()); ?>
                        <?php endwhile; ?>
                        <?php endif; ?>

and in content.php →

<?php the_content(); ?>

ON LIVE PAGE→

FULL SHORTCODE →

function simplisto_the_image($atts) {
    $atts = shortcode_atts( array(
        'to' => 'https://pics.wikifeet.com/Melania-Trump-Feet-720891.jpg'
    ), $atts);
	$output = '<div class="lazyimg">';
	        $output .= '<img class="lazyimg-popup" src="'.$atts['to'].'" alt="The First Caption" width="100%" height="auto">';
        $output .= '<i class="fa fa-expand" aria-hidden="true"></i>';
    $output .= '</div>';
	return $output;
}

function register_shortcodes(){
   add_shortcode('simage', 'simplisto_the_image');
}

add_action( 'init', 'register_shortcodes');