Apple TV Volume control not working properly VIA eARC.

It sounds like you’re taking the right steps for HDMI CEC control…

@Anthony wrote:

[…]

volume up is working but volume down will not work.

This sort of partial function usually indicates the remote needs to be reset, then paired again with the ATV4K.

[…]

this option also allows for control of volume via iOS Apple TV remote. Choosing any of the IR options results in losing volume control functionality via iOS Apple TV remote

From this, are you confirming that volume up and down (and mute) are working correctly with the iOS remote? If so, that’s another indication that the ATV4K, your TV and soundbar are configured correctly (HDMI CEC functions properly enabled everywhere), and that the Siri Remote needs to be reset.

Trouble steps attempted

reset to factory Soundbar, Apple TV and LG TV

update tvOS to 15.4.1

repair Apple TV remote

unplugged all hdmis and power to all devices.

Make sure to do the full reset of the remote as described in here: If your Apple TV remote isn't working - Apple Support. For quick reference:

1. Point your remote at your Apple TV. Make sure that the remote is about three inches from the Apple TV. 
2. Press and hold Back  (or Menu) and Volume Up  for five seconds.
3. If asked, place your remote on top of Apple TV to complete pairing.