Not sure if this is anything anyone can help with, but I’m airing it out there…
Relevant hardware:
Motherboard: ASRock Phantom Gaming-ITX/TB3
GPU: 1080 Ti
Monitor (search on ebay): 13.3 inch IPS 2K HDMI USB Type-C USB-C Portable Monitor for Raspberry Pi PS4 PC
Thunderbolt 3 dock: Belkin F4U109
The primary issue appears to be an incompatibility between the monitor and the Thunderbolt 3 cable I use to connect the dock with, and I found this out through trial and error. Here’s the conditions under which the monitor works:
Connecting directly to the GPU using an HDMI to miniHDMI cable with a DP-to-HDMI adapter on the HDMI end
Connecting directly to the Thunderbolt port on the motherboard using a USB-C cable
Oddly enough, if I try connecting the monitor directly to the Thunderbolt port using the Thunderbolt 3 cable, it doesn’t work. The conditions where the monitor works makes it clear that the issue isn’t the dock, which I first thought, but with the Thunderbolt 3 cable itself. The dock has USB-C ports itself, but those don’t work either. What I haven’t tested is using the USB-C cable to connect the motherboard TB port to the dock, and then try connecting the monitor to the dock, but I’m not sure I have to.
TB3 cables doesn’t automatically support also USB 3.2, USB C, and DisplayPort functionnality specs so that might explain why.
Probably the monitor support only DisplayPort through USB-C, hence why a TB3 cable that doesn’t support that spec (even if the physical connection is USB C) would not work.
And the USB C port on your TB3 dock might only do basic USB 3.2, and not support Displayport through usb-c
It is always best to buy a cable that explicitely supports TB3, USB-C and DisplayPort through USB to reduce the hassles
I decided to test what would happen if I connected a USB-C cable from the motherboard to the dock. The result is devastating. Apparently the dock itself doesn’t support DP through USB 3.2. Tried the USB-C port, USB-A port (front and back), HDMI cable with DP adapter… None worked.
It would appear that going with Belkin was a major mistake. Not sure where to go from here. Maybe I should just settle with RMAs (which hopefully goes through: by some miracle I’ve still got the original packaging for both cable and dock) and some 10 Gbps USB-C dock and cable solution. But what about muh 30 Gbps left behind then? Ugh…
Would a different TB3 dock with the same TB3 cable work though? Perhaps one with an HDMI port instead of DP ports? EDIT: The answer is yes. I went with TB3HDMIDOCK. Obviously DP over USB-C still isn’t supported due to the cable, but the HDMI port works fine.