DIAGNOSIS You are not enabling the monitor's maximum 160Hz overclocked refresh rate. It requires two specific settings: DSC (Display Stream Compression) must be active, and the Overclock setting must be turned on in the monitor's own menu.
FIX First, enable Overclock on the monitor. Press the joystick. Navigate to Game Adjust > Overclock. Set it to On. The screen will blink. Use a DisplayPort 1.4 cable. On your PC, open NVIDIA Control Panel. Go to Change resolution. Under 'Refresh rate:', select '3840 x 2160 (native)'. The list should now include '160Hz'. Select it. Apply. DSC is automatic and requires no user setting.
IF THAT DIDN'T WORK Your GPU may not support DSC. Only NVIDIA RTX 20-series and newer, or AMD RX 5000-series and newer, support DSC for 4K 160Hz. Also, verify your DisplayPort cable is VESA-certified for HBR3 (DisplayPort 1.4). Try a different cable.
HEADS UP When Overclock is set to On, the monitor's response time setting (Fast, Faster) may introduce more inverse ghosting. If you see visual artifacts, set Response Time to 'Fast' instead of 'Faster'. The 160Hz mode also increases power consumption and heat output slightly.