Next & Previous Posts Inside Categories

function zt_front_posts(){
	$ppp = (isset($_POST["ppp"])) ? $_POST["ppp"] : 1;
	$page = (isset($_POST['pageNumber'])) ? $_POST['pageNumber'] : 0;

	header("Content-Type: text/html");		
	$args = array(
		'suppress_filters' => true,
		'category_name' => 'newsletter', //Categoty slug
		'post_type' => 'post', //post type
		'posts_per_page' => $ppp,
		'paged' => $page,
	);
	$loop = new WP_Query($args);
	$out = '';
	if ($loop->have_posts()) : while ($loop->have_posts()) : $loop->the_post();
		$output .= '<div class="posts">';
		$output .= '<h2>' .  get_the_title() . '</h2>';
		$output .= '<p><small><i class="fa fa-calendar"></i>  ' . get_the_date() .'</small></p>';
		$output .= '<p>' . wp_trim_words( get_the_content(), 40, '...' ) .'</p>';
		$output .= '<p class="mt-5"><a href="' . get_the_permalink() . '" class="primary">Weiterlesen</a></p>';	
		$output .= '</div>';	
	endwhile;endif;
	wp_reset_postdata();
	die($output);
}
add_action('wp_ajax_nopriv_more_post_ajax', 'zt_front_posts');
add_action('wp_ajax_more_post_ajax', 'zt_front_posts');

/*
 * Posts Frontpage
 */
 ppp = 1; // Post per page
 var pageNumber = 1;
 var total = jQuery('#totalpages').val();
 jQuery("#more_posts").on("click", function ($) { // When btn is pressed.
	jQuery("#more_posts").attr("disabled", true); // Disable the button, temp.
	        pageNumber++;
	        var str = '&pageNumber=' + pageNumber + '&ppp=' + ppp + '&action=more_post_ajax';
	        jQuery.ajax({
	        	type: "POST",
	        	dataType: "html",
	        	url: the_ajax_script.ajaxurl,
	        	data: str,
	        	success: function (data) {
	        		var $data = jQuery(data);
	        		if ($data.length) {
	        			jQuery("#latestpost").html($data);
	        			jQuery("#more_posts").attr("disabled", false);
	        		} else {
	        			jQuery("#more_posts").attr("disabled", true);
	        		}
	        		if (total < pageNumber) {
	        			jQuery("#more_posts").hide();
	        		}
	        		if (pageNumber > 1) {
	        			jQuery("#prv_posts").show();
	        		}
	        	},
	        	error: function (jqXHR, textStatus, errorThrown) {
	        		$loader.html(jqXHR + " :: " + textStatus + " :: " + errorThrown);
	        	}

	       });
	        return false;
	});

	//Previous Post
	jQuery("#prv_posts").on("click", function ($) { // When btn is pressed.
		jQuery("#prv_posts").attr("disabled", true); // Disable the button, temp.
	        pageNumber--;
	        var str = '&pageNumber=' + pageNumber + '&ppp=' + ppp + '&action=more_post_ajax';
	        jQuery.ajax({
	        	type: "POST",
	        	dataType: "html",
	        	url: the_ajax_script.ajaxurl,
	        	data: str,
	        	success: function (data) {
	        		var $data = jQuery(data);
	        		if ($data.length) {
	        			jQuery("#latestpost").html($data);
	        			jQuery("#prv_posts").attr("disabled", false);
	        		} else {
	        			jQuery("#prv_posts").attr("disabled", true);
	        		}
	        		if (pageNumber < 2) {
	        			jQuery("#prv_posts").hide();
	        		}
	        	},
	        	error: function (jqXHR, textStatus, errorThrown) {
	        		$loader.html(jqXHR + " :: " + textStatus + " :: " + errorThrown);
	        	}

	    	});
	    return false;
	});