1) Try to open a sheet music and see if the icon of device connection is red or green. See the icon on upper left of your smart device screen.
2) If it is red like above, please try to use another smart device and connect your piano with the app, to see if there's something wrong with the cable.
3) If another smart device connection also failed, then a new cable would be needed.
4）After replacing cable, it still cannot be connected, the HDMI port needs to be replaced.