Using Chillispot on DD-WRT v24-SP1:

If you use a redirect URL that contains a GET variable, Chillispot will incorrectly append its own GET variables with a leading '?'.  This leads to the loss of the first GET variable, which appears to always be the 'res' variable.

Example:

If you supply a the redirect URL 'https://www.foozle.com?foo=bar', hotspotlogin.php (or the moral equivalent) will use the URL 'https://www.foozle.com?foo=bar?res=notyet&uamip=...' (incorrect) instead of 'https://www.foozle.com?foo=bar&res=notyet&uamip=...' and so on.

The result is that GET variables cannot be used in the redirect URL, because the hotspotlogin.php (or whatever) will be unable to obtain the value of the 'res' GET variable.