If your monitors stay black, or only one turns on, when connected to the Thunderbolt 5 CubeDock (ST-DT5CES-US, ST-DT5MCC), work through these checks in order.

How to fix a blank or single display on the CubeDock

1. Connect the CubeDock to your computer using the included Thunderbolt 5 cable. A standard USB-C cable may not carry full video and data.
2. Plug the CubeDock into its power adapter, since the dock needs external power to drive displays.
3. Confirm how many external displays your computer supports. On a Mac this depends on the chip: M1 and M2 base chips drive one external display, an M3 base chip drives two but only with the laptop lid closed, and M-series Pro, Max, and Ultra chips drive more. Most Windows laptops also drive more than one.
4. Set your displays to Extend, not Mirror. On Mac, open System Settings, then Displays. On Windows, open Display settings.
5. Reseat each monitor cable, and test one monitor at a time to rule out a bad cable or port.
6. Update macOS or Windows, then unplug the CubeDock for 20 seconds and plug the CubeDock back in.