Sorry - I may have expressed myself inaccurately. The information comes from Apple and works like this for me on Mac Min and Display.
TB3 corresponds to USB 3.1 in terms of connector design, but can transmit much higher bandwidths. The signal arrives at the monitor as Displayport. I briefly reconnected the cables. TB3 (USB 3.1) to Displayport (large connector) and TB3 (USB 3.1) to USB 3.1 on the monitor work for me. The quality of the USB/TB cable is important. The cheap ones (I only have one at hand) do not transmit the signal.
I connected my monitor to the Mac Mini with an OWC TB3 cable LMP THUNDERBOLT 3 (40GB/S) USB-C CABLE 2.0M and everything works. Unfortunately, Digitec does not yet carry this quality of TB3 cable and only has a 40GB, 0.5m cable on offer.
Info about TB3/USB:
https://thunderbolttechnology.net
@digitec: Would not be bad if more TB3 quality cables > 0.5m were on offer :-)