I don’t think you are doing it right. That said, I may have gotten the wrong end of the stick of what you are trying to achieve.
With the Librem 5 and that hub …
- connect the Librem 5’s original power supply to the USB-C socket on the hub
- connect any keyboard and/or mouse using any USB-A port(s) on the hub
- connect the hub to a monitor (or TV) using an HDMI cable
- connect the hub to the Librem 5 by using the integrated USB-C cable of the hub
The Z400 doesn’t really come into it.