Hi Mike,
I don't think you need Chromecast in the chain. Your Naim Uniti Atom should stream directly from your ethernet connection, not through Chromecast. Here's what it says on the Naim Uniti Atom Headphone Edition page from Naim's website:
Easy App control – in one room or many
The Naim App – available for Apple and Android devices – makes it simple to control your Uniti Atom Headphone Edition, and even link up other music systems around your home into a stunning multi-room set-up.
Use the App to access all your music, including your favorite radio stations, streaming services and playlists, and explore artist discographies and biographies.
Uniti Atom Headphone Edition works seamlessly with other Naim streaming products, including Mu-so systems. Choose the same song in beat-perfect sync or play different music in different rooms. You can also pair Uniti Atom Headphone Edition with other AirPlay 2-compatible wireless speakers via the Apple Home app, or with other Chromecast built-in devices via the Google Home app.