If you want two domains on your site to have the same content, then the Parked Domains option is the option to use.
Looking at both your .com domain and your free subdomain, it looks like the HTML content is identical for both sites, so that’s good. However, the HTML content of the page still references the free subdomain, even if you visit the .com domain. And due to the hotlinking/CORS restrictions on our hosting, that results in the broken page.
So in terms of accounts and domains, your configuration is correct. Now you just need to make your website work with it.
I don’t think SMF likes to be used on two domains simultaneously, so the first step is to pick one primary domain to host your site (probably the .com domain), and update your website’s configuration to work with it.
This reminds me of a topic from a few weeks ago from someone who also wanted to move their SMF site from a free subdomain to a custom domain. Maybe it contains useful information for you:
After updating your site to work on the desired domain, you may want to make sure that all other domains redirect to the primary domain of your site. You can do that with a .htaccess snippet like this one: