Thanks for reaching out to us, I am sorry this is not working as expected.
Thank you for providing the monitor model number and system model number!
This monitor supports HDR10 ( 8bit+FRC ) which is most likely causing the refresh rate to be limited to 30Hz. HDR10 requires HDMI 2.0b, our DisplayPort to HDMI adapter supports HDMI 2.0 and does not support HDR10, we can attempt to disable HDR10, if the option is available in the monitor, to get 60Hz refresh rate. Alternatively a miniDisplayPort to DisplayPort cable, instead of our miniDisplayPort to HDMI adapter, may also work to get 60Hz refresh rate. DisplayPort 1.2 ( supported by this system ) has slightly more available bandwidth than HDMI 2.0 and may be enough to negotiate 60Hz refresh rate with this monitor.
The monitor’s manual can be downloaded from LG here ( http://gscs-b2c.lge.com/downloadFile?fileId=kRfrA8tltQZoLMQTwlak9w ) instructions for accessing the on screen display can be found on page 25.
First we can make sure HDMI Compatibility Mode is disabled ( manual page 37 ) under Settings > General. HDMI Compatibility Mode limits the HDMI input to the HDMI 1.4 specification which will reduce the refresh rate to 30Hz.
Second we can set the Picture Mode to Custom in Settings > Picture and look for any options to disable HDR or set 8-bits per pixel or 24bit color. Based on the user manual I do not see these options, however it doesn’t seem to go into full detail on what is within the custom settings.
Please let me know if this helps and we can go from there.