Could you send us a link to the cable you are using?  Or tell us the brand name and model?
 
Because you are a new member, your link will be removed by the forum software. Add some spaces in the link so that it isn't detected as a link, so that the forum lets you post it here.
 
Are you sure that the cable you are using is a display port to USB-C cable, and not a USB-C to display port cable?  They are usually directional.  You might be using the wrong cable in the wrong direction.
 
For example, this would not work, because it is USB-C to display port, not reversible:
https://www.amazon.com/DisplayPort-Certified-Gold-Plated-Thunderbolt-Compatible/dp/B07RY75V7Q
 
But this one should work (maybe?) because it works in either directions and reviews say that it worked for them:
https://www.amazon.com/AmazonBasics-Bi-Directional-USB-C-DisplayPort-Cable/dp/B081VK7Q94
 
But one review says the above bi-directional cable is not always compatible because some display port and USB-C ports don't provide power to the cable, so the circuitry in the cable doesn't power-on.  In such a case, you might have to look for a bi-directional cable which includes a separate power source.