First of all, thank you for the replies .... sure helps a fumbler like me ! I had already cast soundcloud tracks through the TV chromecast and then on through TV optical out to the soundbar.
But then I wanted to see if I could cast soundcloud tracks straight from android phone to the sound bar without turning the TV on. I had not realised the chromecast dongle would already have wifi and due to limited space in soundbar compartment, needed a short HDMI extension wire as the dongle would not fit directly into HDMI in.
Also thought the soundbar would have to be connected in the home wifi network for the chromecast dongle to work. Thus the attempt to add wifi capability to the soundbar thru the soundbar USB port. Two misconceptions cleared !
So it looks like just a direct chromecast connection to soundbar HDMI in would work. No reconfig of chromecast needed ?