I recently installed Ubuntu 20.04.1 on two Asus laptops. Both have the Nvidia 390.138 driver installed and both had a screen tearing problem.
I was able to solve the issue on the GT635M laptop by creating a file
containing the line
options nvidia-drm modeset=1
and then running
sudo update-initramfs -u
I then confirm using
sudo cat /sys/module/nvidia_drm/parameters/modeset
This does not work on the GT650M laptop. I also tried
xrandr --output eDP-1-1 --set "PRIME Synchronization" 1
which does not work either.
What else can I try?