Thought I would share my success, for those in a similar position to me.
Before I start, my setup includes:
65 LG C9, PS4 and 2018 N950 sound bar.
Since getting the C9 I could not get Dolby Atmos to play through my sound bar until I did the following. Ensure eARC is turned on in the sound settings on your TV and set to passthrough not auto or pcm.
If this is working on my model, surely the Q90r sound bar would be the same.