Commenting on that tutorial might get attention of the author and you'd get a better answer.
Is the tracking pixel working if you add index.php
in the URL like this example: https://yourdomain.tld/index.php/mtracking.gif
? If so, then double check that you did step 6.d correctly:
Update the Nginx Configuration to fix a problem with Mautic accessing its assets when a redirect to the SSL site version is enabled.
cat "rewrite ^/index.php/(.*)$ /$1 last;" > /var/www/yoursitedomain.net/conf/nginx/urlfix.conf