Thanks for reaching out, @Stephen_Bayle!
Starting out, may I kindly ask that you please provide a brief description of your current setup using this link as a guide.
Make sure to describe your network configuration/topology, including any networking hardware currently in use, so we can have a clear understanding of how your devices are communicating.
If you reinstall the Roon application on the iMac (just the application — do not delete the database) is there any change in behavior?
Thanks,
Dylan