500 Error – when trying to change the domain name

I was trying to change the domain name of my wordpress site from a .com wordpress site. I didn’t want to carry anything over, just change the domain name. The domain name for the site was purchased directly from wordpress so I thought it would be easier to switch over. I changed the name through the wp-config.php file by changing the current name to
define(‘WP_HOME’,’applicable-name’);
define(‘WP_SITEURL’,’applicable-name’);.
When I changed the name, it simply lead to the other wordpress.com site, not the new one. So that I could at least continue to work on the site in the mean time, I switched it back to its original name. The moment I did that, the 500 and earlier, 404 errors began. That is the only thing I changed, nothing else at all. It is also set back to what worked perfectly fine earlier since all I had to do was change the name. I have no access to the dashboard so debugging isn’t an option and all solutions must exist outside of it.

The default name for my site was the hostname of my computer, I didn’t choose it but that is what it is. I cannot use an FTP client because I have not set it up with one. It is simply being hosted on WordPress.org and I have only had this site for about 2 days. The original setup instructions I followed to create the site were verbatim taken from https://www.digitalocean.com/community/tutorials/how-to-install-wordpress-with-lamp-on-ubuntu-16-04 if that matters to anyone.

Please let me know if you either know how to solve this 500 error or how to transfer the domain name over.

Thank you so much for anyone who can offer any support.