|Your terms are a little confusing. When you say..|
internal server's - Do you mean the private IP like 192.168.x.x
external router's ip - Do you mean the public IP like 188.8.131.52 (or what ever your ISP is assigning your router)
If you want to put a Static Public IP on your WAN side of the router then take the address assigned to you by you ISP and turn of DHCP on the WAN connection and enter the IP address and subnet given to you by you ISP.
If you don't have one assigned and do not want to pay the extra amount for a Static IP then you can use a proxy site which will require you to install an app inside your private network that will send the current dynamic IP to the proxy host so that you have a virtual Static IP. A lot of the Free Domain Name Hosting sites will give you this ability. Just Google Domain Name Host and pick one.