If your using ARC and not eARC for your Arc’s connection point in the TV, and you don’t have CEC turned on, you may need to set the audio for all your sources to Dolby Digital Plus.

Itwould probably be easier, though, to turn on CEC. However, Dolby Atmos will only come from sources that are encoded in it, the Arc doesn’t change data being sent to it in other formats. So, if a source is in stereo, it will play in stereo, not Dolby Atmos, etc. 

I am using arc (my tv is too old for earc). Cec is on. 
 

When i play ie something from Amazon prime which is in Atmos it still shows as Dolby digital 5.1 in the sonos app

When you watch something else, does it show as Atmos? Services have been known to make errors in labeling before.
Have you checked the Amazon app’s audio settings, to make sure it’s supposed to be sending Dolby Digital Plus, rather than just Dolby Digital?

Where does this app reside? Is it possible the app is too old to handle Dolby Digital Plus? 

Have you updated the TV’s firmware?

No Atmos from none of my streaming services (Netflix, Amazon, hbo). 
 

the apps are the TV’s resident apps. Firmware updated

Can you post photos of your TV’s current audio settings?

Test The Tomorrow War on Amazon Prime. If you have a Netflix premium subscription, test Don’t Look Up. If neither one of these play as Dolby Atmos, then more than likely your TV doesn’t support Dolby Atmos audio.

The tv does not support Dolby digital plus, Only pcr, Dolby digital and dts neo 2.5

My TV is in Danish so screenshot wont help :-)

i do have Netflix premium but dont look up just shows as 5.1 inside the Netflix app. Does that mean my tv is too Old? Is there a workaround? Ie chromecast?

When testing Dolby Atmos content, be sure to play the original language audio track.

Your TV is probably too old. Your options are either to upgrade your TV (preferably to a TV with eARC) or get a device like the HDFury Arcana that will allow any external media device to bypass your TV’s limitations and send audio directly to the Arc. This will have no affect on your TV’s native apps though so you will have to get a media streaming device. I recommend the Roku Ultra. A Chromecast would work with the Arcana too but just be sure to use a model that supports Dolby Atmos from all the major streaming apps.

Agreed with @GuitarSuperstar , if the TV doesn’t support Dolby Digital Plus, or Dolby Atmos, you’re never going to get it to send the appropriate signal to the Sonos. And if it doesn’t support it on the internal apps, it’s extremely unlikely to pass it through, so using another streaming box, such as Roku Ultra, Chromecast, or Apple TV, etc, would require the Arcana (or Feintech)  to get the appropriate signals to the Arc. 

The limitation is your TV set, unfortunately. 

Does your TV has audio passthrough or automatic settings available? If so, give them a try.

I think it is VERY unlikely that a TV from 2016 supports Dolby Atmos. And if it doesn't do DD Plus that settles it.

Your choices are to buy a Dolby Atmos compatible TV or ro get an HDFury Arcana device. This  in effect adds eArc and Dolby Atmos to a TV. I use it with my non-Atmos LG TV from 2016.