Answer by Julen for Second monitor not detected, brightness not working... with Nvidia (GTX 1660ti) and Ubuntu 20.04 (kernel 5.4.0)

I could finally make the second monitor work by updating to 5.6 kernel:

wget -c https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.6/linux-headers-5.6.0-050600_5.6.0-050600.202003292333_all.debwget -c https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.6/linux-image-unsigned-5.6.0-050600-generic_5.6.0-050600.202003292333_amd64.debwget -c https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.6/linux-modules-5.6.0-050600-generic_5.6.0-050600.202003292333_amd64.debsudo dpkg -i *.deb

About the screen brightness, this just works partially; the difference between the min and mix brightness is minimal.