RDS: HA Connection Broker, two different clusters?

Currently we have a Windows 2019 cluster running Remote Desktop Services with High Availability roles for Connection broker and VM Host running on this cluster. I have been told by MS support that this is an unsupported setup and needs to be split up. I was told that we could rebuild this system with the Connection broker on another cluster in HA mode. In practice however this doesn't seem to work.

We have a second cluster, different machines, and when I try to deploy RDS roles to this cluster I cannot do so. None of those servers nor their cluster are ever presented as a place I can install to. Am I doing something wrong? Is it possible to run RDS services across two separate clusters?