Well if your router does not support simple port translation i.e. any inbound requests on port 80 are forwarded to a local ip on port 80 you could look at haproxy
You have not made it very clear how your network is set up so I am going to assume your WAN system is what holds the internet connection and external IP.
You can in stall haproxy on your WAN system and set up a conditional redirect, e.g. any requests that come in for example-domain.com on port 80 get forwarded to local IP 192.168.1.25 on port 80.
As you have not said what application it is, what protocol it uses and what OS you have running on your WAN and LAN systems it is quite hard to give you any sort of specific answer.