DIAGNOSIS That's by design, not a fault. The USB hub (the downstream ports on the side and back) only gets its data connection from the upstream USB-B port or the USB-C port. The DisplayPort and HDMI ports carry only video/audio, no data.
FIX To use the side USB ports, you need to activate the hub. Connect the included USB-B to USB-A cable from the monitor's 'USB Upstream' port to a USB port on your computer. Now, the side ports will work, regardless of whether you use DP, HDMI, or USB-C for video.
IF THAT DIDN'T WORK If you've connected the USB-B cable and they're still dead, check the OSD. Go to General -> USB-C Power or USB Hub setting. Sometimes it needs to be toggled or set to a specific mode to enable the downstream ports.
HEADS UP The USB hub shares bandwidth. If you're transferring large files from a USB drive on the side port while also using webcam/mouse, you might see slowdowns. For high-speed devices, plug them directly into your PC.