Hi,
Things I'd check in such a case:
Ensure port 4700 on your Ubuntu server is indeed reachable from the NetXMS server, i.e. there is no firewall in between or on the Ubuntu server blocking access. A simple telnet test to port 4700 should do.
Check if Agent Polls are disabled on the node.
Run a Full(!) Configuration Poll against the node.
Enable some debug logging on the server and/or agent. Run polls and check the logs for potential issues.
Cheers
