UR824 wont output a mike input while loopback is on

Hi, I’d like to play along to a cubase song live and send signal to OBS (and then to Zoom meeting). To get the stereo mix from cubase I believe I need to use UR824 Loopback function. However when I do this the UR inputs do not get into the stereo mix even though there’s meter activity in cubase. I read somewhere that Loopback uses the input 1+2 channels but it also seems top block the others. Is this right and if so would something like VB’s VoiceMeeter Potato be the solution?

many thanks