To use ARC you need to have it in both the TV and soundbar. Sounds like you connected the soundbar to the HDMI-ARC jack on the back of the TV.
You usually have to turn it on in the TV menu. You may also have to turn off the TV speakers and change the audio output to PCM. Turning one CEC in the TV may make using the soundbar easier.
Check the soundbar manual on how to select ARC as your source.