DIAGNOSIS HDMI-CEC control is not active or not configured correctly on either the TV or soundbar.
FIX Enable HDMI-CEC on both devices. On the LG soundbar, ensure 'SIMPLINK (HDMI-CEC)' is set to 'ON' in the soundbar settings menu. On your TV, go to the external device settings and enable HDMI-CEC (may be called Simplink, Anynet+, or BRAVIA Sync).
IF THAT DIDN'T WORK Power cycle the entire system. Unplug both the TV and soundbar, wait 60 seconds, and plug them back in. Allow them to re-establish the CEC handshake.
HEADS UP Some TV brands have limited CEC compatibility. If volume control still fails, use the TV's audio settings to set 'Audio Output' to 'HDMI ARC' and 'Digital Sound Out' to 'PCM' as a workaround.