If you need/want to enter your router via a WAN, you need a statuc IP, if you don't want to pay for one or your ISP won't give you one you can use Dynamic DNS.
Once setup on your router and you have set up a dynamic DNS account: when you get a new dynamic ip it tells your dynamic DNS server your new IP address so you just have to use the name you have choosen. e.g. click here.
You can run web servers and game servers like this too.