subdomain and main website

I’ve created a sub domian called blog.domainname.com and installed WordPress into a wordpress folder

Within the General Settings my WordPress address (URL) is https://blog.domainname.com/wordpress and my Blog address (URL) is https://www.domainname.com

On my hosting server I have a public_html and in there I’ve created an index.php file with the following in it

<?php
/**
 * Front to the WordPress application. This file doesn't do anything, but loads
 * wp-blog-header.php which does and tells WordPress to load the theme.
 *
 * @package WordPress
 */

/**
 * Tells WordPress to load the WordPress theme and output it.
 *
 * @var bool
 */
define('WP_USE_THEMES', true);

/** Loads the WordPress Environment and Template */
require('./blog/wordpress/wp-blog-header.php');
?>

When I type in https://www.domainname.com into my broswer the home page loads, but when I click on any of the pages / posts I get a link broken error

Can anyone tell me what I need to do to fix this

Many thanks