Cannot access admin on new test site located in subdomain

I'm new to Drupal, and want to prepare a test site at a subdomain of my main site. I created the subdomain via my hosting company's online tool, then I copied all the files from my public_html folder to the subdomain folder inside the public_html one.

When I access the test site by going to: http://subdomain.maindomain I get a site that looks and functions like the main one, but I can't log into that test site version of the site. If I try to log in as admin the browser address reverts to the main site instead.

Please advise me whether I set up the test site wrong, and how to obtain a test site on my subdomain that functions in every way like my main one, including the ability to sign into the admin settings.