@ScottMillerNarr
You may need to add an Source Input to the Home Screen of the RokuTV.
Settings..TV Inputs...HDMI 1 or HDMI2 or HDMI3. You can then Rename it to your iMac.
After you add the HDMI Input to your Home screen and select it, your iMac "should" appear.
Here is info how to use iMac for Video Mirroring or extended desktop mode. https://support.apple.com/guide/mac-help/use-multiple-displays-mchl7c7ebe08/mac
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
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