I have same problem, do you have a solution? Can you now use usb-c to usb-c? @MIBrus
If you did not purchase this U2419HC directly from Dell, you need to fill out the Service Tag number Ownership Transfer form.
We can see from your private troubleshooting with our DELL-Cares agents that the U2419HC and its USB Type-C cable (C to C) cable do function on another PC, just not on your Lenovo Ideapad.
As per his thread here and his Lenovo Forum thread, the U2419HC USB Type-C cable (C to C) cable did not work on his Lenovo Ideapad 720S-13. He ended up using a Gembird USB-C to Displayport adapter A-CM-DPF-01 connecting to our U2419HC DP to DP cable.