Many USB-C-to-DisplayPort cables carry video only from a USB-C source to a monitor's DisplayPort input. Plugable explains that connecting a dock's DisplayPort output to a monitor's USB-C input requires a cable designed for that direction, such as a suitable bidirectional DisplayPort-to-USB-C cable.

The guide specifies DisplayPort 1.2 or later support and recommends no more than 1.8 meters for this arrangement. Check the direction explicitly before purchase. Correct connectors at both ends do not establish that the cable can accept the dock's output and feed the monitor's chosen input.