You can use your LAN or WAN IP, shouldn't matter if the port is forwarded to the server correctly. If you can get to your server within your network by using your LAN IP, but not by using your WAN IP, it means the port probably isn't forwareded correctly or something is blocking it.
If you can't access with either, try pinging your server from another computer in the LAN and see if it has connectivity. You can ping by internal IP or computer name (start > run > cmd > type "ping
servername" or "ping
serverip"
The name of the server is the computers name (right click my computer > properties > computer name tab > full computer name). As long as the other computers on your network can "see" your server, you should be able to connect to it by computer name as long as you are within your LAN. Outside of your LAN, you will of course have to use your external WAN IP (unless one of the computers in your network is running a DDNS client such as No-IP, then you can use that subdomain).