I saw this question recently, about making discovery work:

"I am attempting to discover a group of ILOMs which seem to be on a different subnet than their hosts. When I try to discover the ILOMs, Ops Center can't find them. Is it a requirement that the ILOM and its host should be on the same subnet?"

The short answer is no, related assets don't have to be on the same subnet for you to discover them. The correlation between related assets doesn't depend on them being on the same subnet.

What's likely going on, if you're running into similar issues, is that the subnet where you're trying to discover systems needs to be associated with a Proxy Controller that can reach it. You can go here to see how to associate a network with a Proxy Controller. Once the network is associated with a Proxy Controller, you should be able to discover and manage assets on it.