I have noticed the same behaviour - using a USB-C to DisplayPort connection or a USB-C hub (with HDMI). After a while, the external display blacks out and the display reverts to the internal laptop display only (MacBook Air M1, 512GB). What seems to make a difference is whether my MacBook is connected to power or running on battery.
How to Connect to an LG 5K Display. The LG UltraFine 5K Display (Model 27MD5KL-B) requires a Mac with a USB-C port or Thunderbolt 3 (USB-C) port running macOS Mojave 10.14.6 or later, or an iPad with a USB-C port running iOS 12.4 or later.
If you are facing the issue of LG monitor USB ports not working, then there is nothing to worry about, as you can simply follow the above-mentioned solutions. Try updating the Windows, drivers, try using a different USB port, check the size of your USB, and if none of the solutions works, reset the LG monitor.
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