Diagnosis
You are hitting a configuration limit. The Neo G9 57 only supports 240Hz at its native Dual UHD resolution (7680x2160) under specific conditions: DSC must be enabled and you need the correct GPU and cable chain.
How to Fix It
Go into the monitor's OSD Menu. Navigate to System -> DisplayPort Version. Set it to 'DisplayPort 2.1'. Then go to Picture -> Display. Ensure 'DSC' is set to 'On'. In Windows NVIDIA Control Panel, set the resolution to '7680 x 2160' and refresh rate to '240Hz'. Do not use a custom resolution.
If That Didn't Work
Your GPU may not support DP 2.1. Only the NVIDIA RTX 40-series and AMD Radeon 7000-series have full DP 2.1 support. A 3090 Ti or older only has DP 1.4 and is limited to 120Hz at this resolution.
Heads Up
Using HDMI 2.1 ports on any current GPU will max out at 120Hz on this monitor. You must use DisplayPort for 240Hz.