Arc + ATV + (which?) new tv for Atmos

If it were me, and this is only opinion, I’d stay away from TCL and Philips, since they seem to be where most people report substantial issues. But in general, any device that supports CEC properly implemented, through eARC, should be able to handle both versions of Atmos. 90 % of devices handle Atmos via Dolby Digital Plus, which is handled by ARC, so you may be fine anyway. And the amount of Atmos content out there is small, albeit growing. Check your system use before spending money ;)

Find some Atmos content and then check the Now-Playing screen to see if you are getting Atmos to the Arc. If you are I wouldn't use the "no eARC" as a reason to replace a TV I was otherwise happy with.

The AppleTV generally requires an eARC connection to enable Dolby Atmos. There are some TVs that will convert the Dolby MAT stream being sent by the AppleTV to DD+ which works over ARC but those are few and far between.

Apple TV can do it itself, in the Audio settings of the device.

Are you sure about that?

Settings>Video and Audio>Audio Format. 

I have an Arc and a TV that has eARC, but in the case that you only have ARC, it should show Dolby Digital Plus as a format. 

But you won’t get Atmos, as you need to select Dolby Atmos in the AppleTV’s audio settings, otherwise people wouldn’t have bothered buying HDFury Arcana if they could just adjust the AppleTV settings.

If the service provider uses the more common version of Atmos which is carried on Dolby Digital Plus, you should get it without any ‘conversion’, only the MAT data (which, as near as I can tell is mostly Apple content, but that may be changing) will be converted, providing Atmos via Dolby Digital Plus, just not the MAT version. Given the number of speakers Sonos allows in a Home Theater system, I suspect that audibly, there is little difference, but then I’m not an audio engineer, nor do I have ‘golden ears’. 

I’m specifically talking about the AppleTV which the OP has and most likely the reason why they are not experiencing Atmos currently with their setup.

Interesting that you mention staying clear of TCL - I’ve seen many comments suggesting they may be a good pick, though I have some doubts about their overall QC. I do have a lot of atmos content that I’ve bought through Apple as I have a hifi separates setup in my living room for atmos (weirdly, I prefer the Sonos soundstage though, despite the fact that it was a fraction of the cost).

I’ve done that and the app is showing playback is in 5.1

Atmos is not showing as an option, due to the fact that I only have ARC (presumably).

Yes exactly, as only some older TVs from LG and Samsung (that other users have reported) can covert the Dolby MAT output from AppleTV to DD+ (Atmos) which was why I was confused over the suggestions given to find Atmos content or switch audio format when you won’t get Atmos with your current setup.