I turned ''smart" option off on the router. Then plugged cables in to switch one by one and found that all was fine until the dvd cable went in. It won't accept the dvd cable even if that's the only device. Really odd?? But it all seems to work now (excluding dvd, but we don't use internet services on the dvd player anyway).
Thank you to all who gave advice. Much appreciated.
Not really possible as they're very far apart. The switch is 30 meters away from the hub in a summerhouse at bottom of garden and the dvd player is in there. I could bring dvd player in to house and test it but I'd also need to set a TV up near the hub. If I get a chance to do this I'll let you know.