The monitor's energy-saving features are likely disabled in Dell Display Manager or at the OS level.
Enable PowerNap in DDM. Go to the Options tab. Select a PowerNap option to put the monitor to sleep or set brightness to the lowest level when the screensaver activates.
Check Windows power settings. Ensure your PC is configured to turn off the display or put the computer to sleep after a period of inactivity. The monitor follows the PC's signal.
If that didn't work: A video input signal may be keeping it awake. If you have multiple devices connected (like a laptop and a desktop), the monitor might be receiving a standby signal from a different input. Try disconnecting other video cables or manually switching to the input connected to the sleeping PC.
Heads up: The monitor's own power saving features operate independently through DDC/CI. Ensure DDC/CI is enabled (Menu > Others > DDC/CI > On) for DDM's PowerNap controls to work.