src\config\secure.hpp
#define SECURE_NPCTIMEOUT
but I'm not sure if it's going to work. because there is an issue on it which is not fixed for a very long time.
if it's a LAN it doesn't even need to do port forward. all you need to do is to be allowed on your firewall.
and also, on your char_athena.conf and map_athena.conf make sure you use your LAN IP instead of 127.0.0.1 and also check if your client uses clientinfo.xml or sclientinfo.xml and also just use your LAN IP there.
at least I didn't pretend to fix anything which is not really working. if you didn't delete or remove your diff patch, the staff might be. and since the author of this thread is no longer active. this post might also become hidden soon just like what happened on his bg2.0.