If you downloaded LNet using Online Package Manager, there will/should be a tcp example in /Users/<your user name>/.lazarus/onlinepackagemanager/packages/lnet/examples/console/ltcp/lserver.pp
I can confirm this works under High Sierra (although because of installation issues of Lnet, it wants to use the stubs debugger, it was simpler to use Codetyphon which installs the Lnet components without issue).
There is an issue with the Lnet Visual components on a mac, events do not seem to be fired, it was simpler to modify the lserver.pp example and use in a graphical application, than it was to try and find the cause of the visual components failure.
Hope this helps.