Thank you for contacting us, I am sorry this is not working as expected and I am happy to help!
The two most common causes of the HDMI 1 “Alt Mode” output not working are:
- If the docking station is connected to the computer using an incompatible USB-C cable, or connected through another device that does not support USB-C DisplayPort Alternate Mode.
The UD-3900PDZ’s included USB-C cable supports USB 3.0 Data, Power Delivery, and USB-C DisplayPort Alternate Mode, USB-C cables provided with other computers and devices ( for example USB-C “charging” cables like that provided with modern Apple MacBook systems ) only support USB 2.0 data and Power Delivery, but not USB 3.0 data or video.
If you are not already doing so, I recommend using the original USB-C cable provided with the docking station to connect directly from the USB-C port on the back of the docking station to the USB-C port on the MacBook to see if this helps, then try the same with the USB-C ports on the Windows laptops. Please let me know if this helps.
- Not all computers with USB-C ports provide USB-C DisplayPort Alternate Mode video output on these ports. USB-C DisplayPort Alternate Mode is an optional feature of the USB-C specification. If #1 above does not help or does not help on all of the computers, please let me know the manufacturer(s) name and model(s) of the non-functional computers and I will look up the manufacturer’s specifications to verify these systems are fully compatible with this docking station.
Thank you for verifying the docking station is connected directly to the computer with the USB-C cable provided with the docking station, and for the additional system details if that does not help.