DIAGNOSIS You are using an HDMI cable or port that does not support the bandwidth required for 3440x1440 at 165Hz. The monitor's HDMI 2.0 ports are limited to 100Hz at native resolution.
FIX To achieve 165Hz, you must use a DisplayPort connection. Connect your GPU to the monitor using a DisplayPort 1.4 cable. In Windows Display Settings > Advanced Display, the 164.98Hz option will then be available. Ensure Input Signal Plus is 'On' for the DP port in the OSD.
IF THAT DIDN'T WORK If 165Hz is still unavailable over DisplayPort, your GPU may be defaulting to a limited color format. In NVIDIA Control Panel, under Change Resolution, set Output Color Format to 'RGB' and Output Dynamic Range to 'Full'.
HEADS UP The monitor's 'Low Input Lag' mode in the OSD Game menu must be set to 'On' to enable the high refresh rate pipeline. If set to 'Off', it may cap the available refresh rate options.