ATV is connected via Theatre - not LG TV directly. I tried that, but it introduced it's own problems and I seemed to be fighting against the system.

Lan cable still connected between Theatre and TV.

CEC turned on everywhere.

Sorry to say it,

but the ‘normal’ procedure with what you are aiming at, is to connect the ATV via one of the HDMI ports to the tv and connect the soundbar/the theatre via the HDMI/e-ARC port to the tv.
With CEC enabled everywhere!

You do not need the LAN connection from the Theatre to the LG TV….since you are not controlling the tv with the B&O remote.
You do not need the B&O app on the tv either!

As I understand it, you want to control the ATV, let that turn on the LG TV to a specific HDMI port (with the ATV connected there) and also the Theatre.
As I understood your post you are not interested in controlling the tv, the built in sources there etc etc, nor do you want to choose/control any of the built-in sources of the Theatre.
(Should you want to do that, you can use the Bang & Olufsen app e.g. to choose a netradio station).

This is a straight forward procedure for any combination of modern tv’s, soundbars and an AppleTV controlled with the SIRI Remote.

If you want the full B&O experience, you should connect the ATV to the Theatre, connect a PUC cable also, connect the Theatre via HDMI/(e-ARC), install the B&O app on the LG TV and also the LAN cable and turn off any CEC….
…..and then control everything with a BeoRemote One.

 

MM

Thanks. I might try it again to see if it works. What I hadn't done before is disconnect the LAN cable.

One of the reasons I didn't do with this setup in the past is that I want Apple Music Dolby Atmos sound to play through my Theatre via the Apple TV. Do you know if it will do that if the path is ATV > LG > Theatre?

I have my AppleTV connected to my Theatre and the Theatre connected to the TV with eARC.

I have PUC connection between the Theatre and AppleTV and LAN between Theatre and TV to get Beoremote 1 to work. My installer initially turned of CEC and had a CEC blocker box from the AppleTV, as I previously mentioned.

However, as I preferred using the AppleTV remote for most navigation I have thereafter turned on CEC and removed the blocker box. Now I use the Beoremote 1 for changing sound settings and start favourites but the Apple TV remote for most daily usage, the best of two worlds… 😁 So let us aim at replicating my setup as then both the AppleTV remote and Beoremote 1 (as you have one and it is good for sound settings) will work.

I also get Atmos from Apple Music in AppleTV. So you do not to rearrange your cables as I understand it you have your cables like me. Then it is just a question of settings. That we hopefully should be able to solve.

After having to factory reset my Theatre once I got exactly your problem, but after some work with all settings I managed to get it to work. Though it was not obvious and took some time to sort out. Unfortunately I have forgotten exactly what solved it.

Do you have a PUC connection today between your Theatre and AppleTV?

I also think my AppleTV changed the settings once after a tvOS update. Let us start with securing that you have correct settings in AppleTV (unfortunately I have Swedish interface so my translations might be a bit wrong): 1) In Settings select “Remote controls and units” 2) Go to “Home cinema settings” In the bottom om the list 3) Secure that “Control TV and receiver” is set to “ON”, that “Volume control” is set to “Auto through HDMI (receiver)”, and finally that “Show volume at screen” is set to “Hide”.   Attached screenshot show my settings in the B&O app for the PUC. I don’t think there are any other settings in the B&O app that is related to this. if you have your AppleTV and B&O set as above we can continue with LG. Though some of their settings are quite hidden.  

SOLVED - I went back and tried Apple TV > LG > Theatre, rather than Apple TV > Theatre > LG and that works just fine. I didn't need to disconnect the Theatre<>LG LAN, so the Beoremote works if I need to control the TV settings, but for everyday use, I can just and only use the Apple TV Remote, including the correct power-on behavior.

Thanks Bill for the prompt to try this again.