How do I get Dolby vision settings to work? My tv has it.

What is the model TV that you connect to?

Do you have the 2017 model (A1842) or the 2021 model (A2169) of Apple TV 4K? Identify your Apple TV model - Apple Support

Both can do DV, but there are some differences:

• The 2017 model has an HDMI 2.0 port, requiring a 18 Gbps HDMI cable for some content, like 4K DV 60Hz 4:2:0. Note that you need to set 4:2:0 chroma to allow the other video properties at high settings.
• The 2021 model has an HDMI 2.1 port, requiring a 48 Gbps HDMI cable for some content, if you other equipment also has HDMI 2.1 ports. Then you may get away with e.g. 4:4:4 chroma (even though that doesn’t improve anything, as all source videos are encoded with 4:2:0 chroma anyway).

So check your video settings and your HDMI cable speed rating.

Use HDMI 2 or 3 on your TV for 4K HDR 60Hz compatibility. I read that HDMI 1 and 4 may be limited to 30 Hz.

The TV has HDMI 2.0 ports, so no HDMI 2.1 features for you with this screen. An 18 Gbps HDMI cable will be sufficient; an 48 Gbps cable will also work, but will never exceed 18 Gbps anyway with this TV’s HDMI 2.0 ports.

Remember to configure the HDMI port in use for Enhanced Format, on the TV.

Could be a cable issue or maybe your ATV4K connects through a soundbar or receiver that doesn't (or isn't set to) support HDMI v2.1a.

Suggested reading: About 4K, HDR, and Dolby Vision on your Apple TV 4K - Apple Support

please tell me what features I would get with HDMI 2.1?

HDMI 2.1 allows for more bandwidth (up to ~48 Gbps), allowing device features such as 8K/10K, VRR, eARC, HFR, QMS, 4:4:4 chroma in combination with 4K HDR 60Hz, for compatible devices.

It requires HDMI 2.1 ports on devices on both ends of the cable (or throughout the chain, depending on connection layout with more than two devices), and an Ultra High Speed 48 Gbps HDMI cable.

HDMI.org

HDMI 2.1 - Wikipedia