DIAGNOSIS The USB-C connection is not sending a proper wake signal. This is a common compatibility issue between macOS and this monitor's firmware.
FIX Update the monitor's firmware. Download the latest firmware file from Dell's support site for the S2722QC. Follow using a Windows PC connected via USB-C or the upstream USB-B port.
IF THAT DIDN'T WORK Change the USB-C Power Savings setting. Press the joystick. Go to Menu > Display > Power > USB-C Power Savings. Set it to Off. This keeps the USB controller more active.
HEADS UP If you use a docking station between the Mac and monitor, the dock may be blocking the wake signal. Connect the monitor directly to test.