Putting any other factors aside for the moment, the model monitor you mention having has an option setting within the on-screen display called, "HDMI ULTRA
HD Deep Color" This setting must be 'On' in order for 4K@60Hz to work via HDMI input. LG calls this out in page 23 of the manual for the display here ->
https://www.lg.com/us/support-product/lg-27UD88-W