I ran into this same problem with a new HP Omen laptop with a RTX 2060.
I followed the instructions of werneR above (using the Software & Updates GUI), but it still did not work.
Eventually, I realized the issue was the SecureBoot. On reboot, make sure instead of hitting "Continue Boot", that you choose "Enroll MOK" and give the password you set for SecureBoot when using the GUI. If you do not do this, the driver changes will not take effect (the whole point of SecureBoot), and your second monitor will not work.