eARC HDMI cable from tv to Arc, for physical connection and an Atmos signal from your tv. Do you have surrounds? Era 300’s have up firing speakers for best Atmos capability. 

Don’t think my tv has eArc. Would I need to use an eArc adapter? I have 2x play one at the moment and a sub. I would upgrade to era 300 at a later stage.

This information is given on Sonos’ web site. Look at the Arc details and you’ll see:

Experience Dolby Atmos home theater content with a compatible TV and HDMI eARC or HDMI ARC connection. Refer to https://support.sonos.com/article/listen-to-dolby-atmos-on-sonos for more information.

  Atmos music from Apple Music or Amazon Music is awesome particularly with classical orchestral, but it’s not perfectly gapless if that matters to you.

 I’m using Arc - Sub 3 - Era 300s.

Is this you? It looks like it only supports 5.1. Pretty old TV now.

https://www.rtings.com/tv/reviews/samsung/nu7100