Help. Lgb8 arc and Denon s216 soundbar

[Alfa 1971]

Hi all. I hope someone can help before I go mad. I've purchased a Denon S216 soundbar and I can get it to work via optical but not Hdmi arc. I have an Lgb8 oled, I've made sure cec is on and the TV is on the correct settings but it's not working. Can anybody solve for me please. Thanks