On the lfs insim server you use one port only. It handles all connections. Each client connection (per source ip) to the server needs to have its own port.
i.e, you start a dedi client and type '/insim 29999' in it.
Then you start one client that connects to that server's port 29999 from its port 30000.
Then another connection from 30003 to 29999 etc.
i.e, you start a dedi client and type '/insim 29999' in it.
Then you start one client that connects to that server's port 29999 from its port 30000.
Then another connection from 30003 to 29999 etc.