In Redirects I chose the required domain name, and clicking the “Alter Redirects” button, there is a minutes-long delay before an error page loads saying " 504 Gateway Time-out".
That’s very strange. I just tried it on your account and see the same error message you see.
It’s strange because the document root of your website definitely exists.
The Redirects tool in the control panel is a bit clumsy and sometimes has weird errors. That’s why I built a different Redirects tool in the client area that should be more sturdy.
In the mean time, you can remove the redirect from your website by hand.
Open the file manager and navigate to the folder of your subdomain.
Open the file called .htaccess in the editr.
Find the lines that start with redirectMatch. These are the redirects setup by the control panel.