Please note that we now use purchase code to validate your theme and Conerstone's Purcahse Code is different from Xtheme Purchase Code. If you have an xtheme license, you don't need to purchase Cornerstone it comes bundled with the theme.
I guess you have a really old version of the theme, if that is the case, you need to manually update your theme and cornerstone using FTP
See Manual Updates Section on the link below
After updating your theme, you can validate it using your purchase code.
For troubleshooting you may refer to this link.
Troubleshooting - https://theme.co/apex/forum/t/troubleshooting-validation-connection-issues/201