You really don't need to turn off your integrated Graphics. As soon as you install the RX 570 GPU Card, BIOS will automatically make the GPU card the main Graphics display rather than the integrated Graphics. Unless you have in BIOS specifically set for Integrated Graphics rather than on auto or PEG or something similar.
You must have the HDMI cable connected to the GPU Card to get HDMI signal to your Monitor/TV. If you have it connected to the motherboard you won't get any signal since the integrated graphics is disabled.