Its a routing problem...Ill try to describe it as good as I can.
Inside your network (you - router/modem) you got an ip...Most often 196....
Outside your network (modem/router - ISP - Internet) you got another ip...whatismyip.com
To be able to open your server, java must check out if the ip you set on configs its the real one. But...java dosent know you got 2 ips (internal and external). So you have to set your internal IP as external. This way java will send the packets to yourrouter and then the router and ISP does the job. If you set your external ip , java wont be able to find it on your local network and you crash.
Fine till now...You managed to make your server online...But there still is a problem. You cannot connect. Why?Because if you set your external IP on l2.ini (as the other players have it) Lineage wont find it on your local network...If you set 127.0.0.1 it wont work either because server ip is 196...And the dumb socket has no idea that 196.. and 127.0.0.1 are on same computer
Conclusion?...You cant play on the computer you host the server, if youre on a router. There might be a way...But I dont know it and it looks like noone else does