I don't use the 390xx driver, but try this:
sudo dnf --enablerepo=rpmfusion-nonfree-updates-testing \
install akmod-nvidia-390xx xorg-x11-drv-nvidia-390xx \
xorg-x11-drv-nvidia-cuda-390xx
That's the driver I normally use but I tried multiple times to install
and uninstall without any success.
Thanks
F