DIAGNOSIS The downstream USB-A ports on the monitor require an active upstream connection to your computer. This is not established.
FIX You must connect the monitor to your PC using either the USB-C cable (which carries video, power, and data upstream) or the separate USB-B to USB-A cable included in the box. For the USB-B cable, connect the square USB-B end to the monitor's 'USB Upstream' port and the standard USB-A end to a USB port on your computer. The USB-C connection handles this automatically.
IF THAT DIDN'T WORK Check the On-Screen Display. Go to Menu > USB-C / USB-B and ensure the correct upstream source (USB-C or USB-B) is selected to match the cable you are using.
HEADS UP The USB hub only has power when the monitor is on, not in standby. Devices may disconnect when the monitor sleeps if your PC's USB power settings are aggressive.