Alexander V. Makartsev composed on 2021-01-10 05:22 (UTC+0500):
...so you need to install "nvidia-legacy-390xx-driver"...
Most likely that's false. As you should be able to see above, old
NVidia cards don't require tainting software to run X on modern
widescreen displays at their native resolutions. Almost certainly
all OP /needs/ to do is eradicate the tainting software, and its
configuration files, and if applicable, any library modifications
its installation created.
I suggested to install right version of nvidia driver simply because
OP is already opted in for nvidia driver and installed 418 version.