Apple tv4k 2021 version no atmos on my Samsung HW-K950

How do you connect?

① Apple TV 4K →[HDMI]→ TV →[HDMI]→ soundbar

 or

② Apple TV 4K →[HDMI]→ soundbar →[HDMI]→ TV

 or

③ Apple TV 4K →[HDMI]→ Arcana adapter →[HDMI-video]→ TV

↘︎[HDMI-eARC audio]→ soundbar

If you want to use the TV’s audio passthrough (①), then the TV needs to support eARC (not just regular ARC), else no Atmos from Apple TV will be available.

If you want to use the soundbar’s video passthrough (②), then the soundbar needs to support the video format (e.g. 4K HDR), else video specs will be limited.

Apple TV 4K outputs Dolby Atmos in Dolby MAT format (uncompressed). Not all Atmos-enabled audio systems can handle Dolby MAT. Even if the bandwidth is no issue, this specific format may not have been implemented widely when your soundbar was designed (just not sure). The MAT specification was pretty new then. That (‘Dolby MAT’) is the question you should be asking Samsung Support.

I couldn’t find if this soundbar has HDMI 1.4 or HDMI 2.0 ports. The fact that it wasn’t publicly specified, suggests to me that it likely has HDMI 1.4 ports (thus no 4K HDCP support). For use with 4K TVs, setups ①③ would then be better for video.

In the Apple TV 4K settings for Audio, check if Audio mode is set to auto or 16-bit. Some TVs expect 16-bit audio and perhaps your Samsung mentions this in its user guide. Also, that HDMI 2.1 cable should explicitly state that it supports Dolby Vision, Atmos, and eARC (your soundbar). The Belkin Ultra HD 4K HDMI 2.1 48 Gbps cable does not mention Dolby Atmos support (unless Belkin confirms it). Other HDMI 2.1 cables available from Amazon do mention Dolby Atmos support.

I have the 2021 Apple TV 4K unit with the mentioned Belkin cable connected to a really cheap 55-inch TV that does not have Dolby Atmos, so cannot further confirm settings.

When did you buy the HW-K950? If you just bought it new (I still see new units on sale even though it's the 2016 model), return it immediately and try a pair of the new HomePod 2s :-).

Assuming you bought it a while ago, I hope the firmware update you're applying improves things. You definitely want your ATV4K directly connected to the soundbar with that new HDMI cable (Urquhart1244's option #2), with another decent cable connecting the soundbar to your TV. Best of luck.

FWIW, I've got an old Sony HT-Z9F soundbar (announced in early 2018), and Dolby Atmos works OK with it.