Ecommerce Shopify WordPress Discussion

Not able to access admin area after changing site URL

So I'm having an issue after changing my site URL to make my site live. For context, I built a site in a sub-folder on my main site. I usually do this and then create an addon domain for the relevant TLD in cPanel to link to the site once it's ready to go, then update the URL in the database to reflect the new TLD. I don't usually have any issues with this, and once the addon domain is established the site will operate at the new address as normal, including access to the admin area of the site. For example: Site developed at: https://main_site.com.au/newsite.com.au Once complete, I create an addon domain in cpanel for https://newsite.com.au with its root folder set to https://main_site.com.au/newsite.com.au and update the database site URL to https://newsite.com.au. I've done things this way without any issues multiple times, except today. For some reason when I make this change, I'm now getting the error "Sorry, you not allowed to access this page." when logging into the admin side of the site. If I change the site URL back to the subfolder, I can access the admin panel without issue, but not if it's set to the TLD. The front end of the site is running without issue, everything is working as expected and I'm not getting anything in the error logs, it's just this issue which is causing me problems. I can't access the backend from the original URL as it just reverts to the TLD URL when I log in (ie. logging in at https://main_site.com.au/newsite.com.au/wp-admin takes me to https://newsite.com.au/wp-admin and displays the same error). I'm at a bit of a loss as to what could be causing this, does anyone have any ideas? Or is there a better way I should be doing this to make sure I don't get this error? Thanks! What I've tried: I've completely reset the .htaccess file - no change. I've checked permissions on the folders (set to 755) and files (set to 644) - no change (I also don't suspect a permissions issue as I'm able to access the admin area when the URL is changed back to its original state). I've deactivated all plugins and themes, and even with everything disabled I'm still getting the error. I've confirmed the correct entries in the wp-users and wp-usermeta in phpMyAdmin, again though, as the error disappears when the URL is returned to its original state, I can't see how this would be the issue. I've tried restoring the site from a previous backup - no change. I've tried changing the 'siteurl' and 'home' entries in the database both through the WordPress site and phpMyAdmin, both end with the same result. Updated to the latest PHP version - no change. Checked database prefixes haven't changed (which they wouldn't have as I didn't physicially change the location of the files or database) - no change.
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