Need more information. Can you provide screenshots or logs? What happens when you use that discovery address? The server returns an endpoint URL you can't resolve? What version of Ignition are you using?
I fixed it myself. It turns out ignition sticks to the discover from OPC UA server. So it inisists to use the port 50000 which is only exposed to service object in k8s. The node port is different. So the connection will be refused.
The fix is make sure node port and container port of the OPC UA server container is the same. Then, it works without issues.
If the server returns an endpoint URL that has a different host and port, it should use those instead, but then both that host and port must be reachable by external clients. Otherwise you would use the host override setting to force correct them.