Issue and Solution.
When you relocate your Roon Core to another device you may find that ARC won’t reconfigure to the new machine. This is likely to be port collision in UPnP. You can clear the UPnP but if you have other unrelated connections then that will become annoying; so just change the port on the ARC settings in the 10,000 plus range that isn’t already open.
This was particularly a problem for me with a Netgear D7000 but it will be common.