Basic domain/URL question

I know it’s basic but I still keep goofing it up. I designed a new WordPress site in a subdirectory of a domain I own (mycustomdomain.com/sites/sitename). I did all the work on it by going to that URL.

The site is now done and I’m ready to point the real domain at it, which I did at the registrar.

So now when I go to newsitename.com, it simply redirects to the full path (mycustomdomain.com/sites/sitename).

I thought the solution was to go into WP’s Settings > General and update the WP Address and Site Address to newsitename.com. But when I did that, I broke the site.

I had to go back in via FTP and manually hardcode the wp-config like this:

define('WP_HOME','https://mycustomdomain.com/sites/sitename'); define('WP_SITEURL','https://mycustomdomain.com/sites/sitename');

So now I’m back to where I started. Can someone please help me understand how to update it all so that it’s now all at newsitename.com?