I guess DDM/SNA is still supported. You should be able to use SNA over IP. However, IP is preferable. You may have security issues. Check CHGDDMTCPA and check for the password required parameter.
If “a remote host refused an attempted connect operation”, then the first thing to do is ensure that the server is even started. (STRTCPSVR *DDM) Next, ensure that nothing is blocking the route to the necessary ports — firewall? router? IP packet filter?