Set up Surround Sound - no sub?

Hi, Although I have had Cubase a while, I am very rusty and have not used Cubase 10 which I have installed. I recently installed a new Audio Device (Focusrite Scarlett 18i20 3rd Gen). I am trying to get this all set up in Surround. So far I am getting 5 of the six channels working correctly, but the sub is not working. I know the cabling works as if I swap cables at hte amp the sub sounds.
I seem to remember there was something in Control Room which needs to be enabled, but I an not sure what… Has anyone any ideas please?
I have Audio Connections set up so that the Outputs tab shows the connections in traditional order - 1=front left, 2=front right, 3= Centre 4= LFE 5=Left Surround, 6= Right Surround. I am not sure what to do with Control Room. I can see activity in the sub in control room but no sound

Thank you all