DIAGNOSIS Your GPU or cable isn't negotiating the full HDMI 2.0 bandwidth required for 4K at 60Hz. This monitor needs HDMI 2.0 for that spec over its HDMI ports.
FIX Go into Windows Display Settings > Advanced Display Settings > Display Adapter Properties for the monitor. Click 'List All Modes' and manually select a 3840 x 2160 resolution at 60Hz. If it's not listed, you need to check your GPU. For older NVIDIA cards, you may need to go into the NVIDIA Control Panel and change the output color format to YCbCr420 under 'Change resolution' > 'Use NVIDIA color settings' to enable 4K60 over HDMI 1.4 (which reduces chroma subsampling).
IF THAT DIDN'T WORK Your HDMI cable is the prime suspect. You need a certified Premium High Speed HDMI cable (often labeled with an official certification label). Many older or cheap cables are only High Speed and max out at 4K30. Swap the cable. Also, try a different HDMI port on the monitor.
HEADS UP For guaranteed 4K60 with full RGB color, use the USB-C input if your PC supports it, or use a DisplayPort cable. The DisplayPort on this monitor handles 4K60 without any special tweaks.