phpseclib always produces login failed

I'm not sure if this is me doing something wrong or whether the server just doesn't support phpseclib or what, but I am trying to connect to an FTP server that will only accept secure FTP connections, and only from certain IP addresses. Now I know that the FTP server accepts a connection from my VPS server as I have remotely connected into my VPS via SSH and manually connected to the FTP server from the command line successfully.

However when I try the following with phpseclib it always hits the exit point.

set_include_path(get_include_path() . PATH_SEPARATOR . 'phpseclib');
include 'Net/SFTP.php';
$sftp = new NET_SFTP('***');
if(!$sftp->login('***', '***')) {
    $sftp->getSFTPErrors();
    exit('Login Failed'); // <-- Code always gets to this point
}
echo $sftp->pwd();

I am unsure on how to even begin debugging this problem. I have asked a question on their forum as well to try and get help. I have tried to enable logging with define('NET_SFTP_LOGGING', NET_SFTP_LOG_COMPLEX); and then output echo $sftp->getSFTPLog(); just above the exit, but that doesn't output anything.

On the page I have just Login Failed, so I'm not getting any errors from $sftp->getSFTPErrors();.

I am completely new to using a secure FTP connection in PHP and don't know what I am doing wrong. Can someone please help me debug the issue?