DKMS should be installed automatically once you install the nvidia driver using Software&Updates or using apt. Installing the non-dkms driver version requires manual intervention. Please post the output of
dkms status
Otherwise, it might be that the nvidia driver is installed but not added to the initrd:
https://forums.developer.nvidia.com/t/linux-mint-nvidia-driver-loads-with-startx-but-not-on-initial-startup/168262/2