I have Roon playing on a Mac Studio through an Anthem AVM 70. Anthem has the ability to input audio and video separately for each HDMI input. For example, say I’m using HDMI one.
On the Roon, I have an Orchard Pecan Pi Dac playing the audio in the HDMI one to an Anthem RCA input which it does well. The video for HDMI one is set to a Chrome dongle playing on the Anthem.
On the Roon setup I have set the Chrome dongle as the Display device for the Orchard Pecan Pi.
Can‘t personally help with Anthem and chromecast, though.
Now, if you‘d also supply all the additional information, as suggested by the support template, some other user or even support, once they get around to it, may be able to distill some useful suggestions.
Apologies for the slow response here, are you still having issues here? What is the exact model of the dongle that you are trying to use? Does the dongle work when bypassing the Anthem and connecting directly to a TV or Monitor?