Diagnosis
Windows is not detecting the monitor's full refresh rate capability due to a cable or driver limitation. The EX2710Q requires specific conditions to run at 2560x1440 @ 165Hz.
How to Fix It
You must use the included DisplayPort 1.4 cable, or a certified high-speed HDMI 2.0 cable, connected to a compatible GPU port. In Windows, open Display Settings > Advanced Display. Click 'Display adapter properties for Display 1'. Navigate to the Monitor tab. In the 'Screen refresh rate' dropdown, 165Hz should now be available. Select it and apply.
If That Didn't Work
Your graphics card drivers may be defaulting to a generic monitor profile. Update your GPU drivers from NVIDIA or AMD. After updating, open the NVIDIA Control Panel or AMD Radeon Software. Create a custom resolution profile with 2560x1440 and a refresh rate of 165Hz, ensuring the timing is set to 'Automatic'.
Heads Up
If using HDMI, note that 165Hz is only available with Adaptive Sync (FreeSync Premium) disabled. To enable 165Hz over HDMI, you must disable Adaptive Sync in the monitor OSD under Gaming > FreeSync Premium.