AW2518HF, GTX 1060, does not reach 240Hz

I bought a AW2518HF but have a NVIDIA GTX 1060 and an i7-7700HQ Alienware 13 R3 laptop.  I can get the monitor to work up to 144Hz but it is not letting me put it in 240Hz. It will let me select that but they the screen goes black on the external monitor and won't come back up until i switch it back.  I am trying it over the HDMI cable provided. I am going to buy a display port cable to see if that helps but do you have any idea why this is not working?

Thanks.