In order to access your MediaWiki via the server URL (http://servername/~user) instead your domain, you should edit your LocalSettings.php file. It is located in the root directory of your MediaWiki. You can edit the file from:

cPanel-> File Manager

Replacethe following line:

$wgScriptPath= "/wiki"

with this one:

$wgScriptPath= "/~username/wiki"

Then you can use the following link to access your MediaWiki:

http://servername.com/~username/wiki/