DIAGNOSIS The downstream USB-A ports get their data and power from the upstream USB-B or USB-C connection. If that upstream link isn't active, the side ports are dead.
FIX You must connect the monitor to your computer with either the included USB-B to USB-A cable (using the square USB-B port on the monitor) OR the USB-C cable for video. If using USB-B, connect the square end to the monitor's 'USB Upstream' port and the regular USB-A end to a USB port on your PC. The monitor must be on and the PC must be on for the hub to activate.
IF THAT DIDN'T WORK If connected via USB-B, check your PC's Device Manager for any unknown devices or issues with USB controllers. Try a different USB-A port on your PC. If connected via USB-C, ensure the cable is fully seated and try the other USB-A ports on the monitor in case one is faulty.
HEADS UP The USB-A ports only provide standard USB 3.2 Gen 1 power (5V/0.9A max). They're fine for peripherals but won't fast-charge a phone. Also, they only work when the monitor is on and connected to a source.