What you need to do to assign the IP address, is configure NAT (Network Address Translation) on the Internet router, or on your firewall appliance. This will change the address that your server uses, and send out the packets to the Internet using the fixed IP address allocated by your ISP.
If you do not manage this router, then maybe they will do this for you. If you do manage it then the manual that came with it should have the details of how to configure this option.
An alternative, if the server is the only system that needs to access the Internet, is to install another NIC card, and apply the fixed IP address to that card. The default route on the server should then have the ISP router as the destination, and any internal networks routed with static routes through the original NIC.
If you are connecting your server to the Internet, make sure you have some firewall in place, and only allow in the traffic that is absolutely necessary. For example, if it is a webserver, only allow http and/or https.