Ok, ive noticed a lot of people are having problems with this, and that they cant decrypt the l2.ini file with any decrypt tools. So, i decided id search around the internet for a decrypted l2.ini file (as im making my own server aswell). I finally found one, so i will upload it here for anyone who needs it. When youve downloaded it extract it to C:\ProgramFiles\NCsoft\Lineage II\system. Then browse to that path open it up and put in your external IP that you have put in your server.properties or login.properties file. You should be able to connect to your server now.
Enjoy