DIAGNOSIS Windows is defaulting to a basic display driver, which means you're probably not using a DisplayPort cable, or you're using an old HDMI standard that doesn't have the bandwidth for high refresh rates at QHD.
FIX First, verify your cable. You MUST use the DisplayPort cable that came in the box, or a certified high-speed DisplayPort 1.2 (or later) cable, to get 144Hz at 2560x1440. If you're using HDMI, you'll need an HDMI 2.0 cable and port on your GPU to get beyond 60Hz at this resolution. After swapping the cable, go to Windows Settings > System > Display > Advanced display settings and select the monitor, then click 'Display adapter properties for Display 1'. Go to the 'Monitor' tab and select 144Hz from the screen refresh rate dropdown.
IF THAT DIDN'T WORK Your GPU driver might be out of date or corrupted. Do a clean install of your latest NVIDIA or AMD graphics drivers. Also, check the monitor's own OSD: Go to 'Game Setting' > 'Overclocking' (yes, it's oddly named here). Make sure 'Refresh Rate Num.' is showing 144Hz. If it's not, the monitor isn't receiving a high-bandwidth signal.
HEADS UP If you have multiple monitors connected, Windows sometimes limits the refresh rate options on all displays to the lowest common denominator. Try disconnecting other monitors temporarily to see if 144Hz becomes available for the CQ27G2.