In very simple terms what you want to do is have the ISP ROUTE your NEW IP Range to the Firewall on the Firewalls Current IP.
You can then create a Node for the Windows Server with the Private IP and then under Static NAT have the Public IP that want it known as.
At the moment when you do that then because the ISP Router has a Local Subnet for your Public IP then it does an ARP who has for the IP.
Nothing responds as doesn't exist and so you never get a reply.
If you provide the Check Point's External IP to your ISP and have them route the new Public IP Range to that IP then that will have the ISP Router forward all traffic for your new IP Range to the Check Point.