I understand you are trying to use 4K screen resolution at 50Hz.
Let me apologize for any inconvenience this issue may be causing to you.
In order to help you better I would like to get the Intel Graphics Driver Report. This link will show you how to generate it http://www.intel.com/content/www/us/en/support/graphics-drivers/000005848.html
I was also checking the specifications of the monitor on LG’s website, but I did not find information about if the monitor supports or not 50Hz. Where did you see the monitor supports 4k@50Hz?
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The monitors EDID is reporting only 60Hz via DP but via HDMI it is different, which is why I asked here why custom resolution cannot be set.
Thanks for the information you sent to me.
According to the Intel Graphics Driver Report the monitor does not seem to support 4K@50Hz. We can try a couple of troubleshooting steps, but there is a chance you may not be able to get 4K@50Hz.
One of the troubleshooting steps you can try is updating the graphics driver. You can download latest graphics driver from this link https://downloadmirror.intel.com/26669/eng/win64_154516.4627.zip
Please install the graphics driver with the steps that are on this YouTube video https://www.youtube.com/watch?v=S1KzoQDjMMY
The content on the above site is not controlled by Intel. This information is offered for your convenience and should not be viewed as an endorsement by Intel for the merchants or services offered there.
Another troubleshooting step you can try is installing the monitor driver so they system can properly recognize all of the monitor features. You can download the driver from this link http://www.lg.com/uk/support/support-product/lg-27UD68P
Let me know if issue persists or not.