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

I think there is a missing file in the previous post : linux-headers-xxx-genericMainline kernel installation needs 4 files and not only 3.

So try 5.10.4 kernel (latest today) with :

wget -c https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.10.4/amd64/linux-headers-5.10.4-051004-generic_5.10.4-051004.202012301142_amd64.debwget -c https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.10.4/amd64/linux-headers-5.10.4-051004_5.10.4-051004.202012301142_all.debwget -c https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.10.4/amd64/linux-image-unsigned-5.10.4-051004-generic_5.10.4-051004.202012301142_amd64.debwget -c https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.10.4/amd64/linux-modules-5.10.4-051004-generic_5.10.4-051004.202012301142_amd64.debsudo dpkg -i *.deb