I created a new user. However, after logging in as the new user, the Monitor Settings cannot detect my monitor and therefore cannot set the proper resolution. When I log in with the old user, the monitor is detected and the proper resolution is used. I am using Ubuntu MATE 18.04 with the NVIDIA drivers.
Adding the new user to the video group and rebooting fixed the issue.
Never mind, the real problem is that only tty7 detects the monitor. Other ttys can't detect the monitor.