This has changed significantly from RHEL 6
Here ae the steps:
GRUB_CMDLINE_LINUX=“rd.lvm.lv=rhel/swap crashkernel=auto rd.lvm.lv=rhel/root ipv6.disable=1”
* Run the grub2-mkconfig command to regenerate the grub.cfg file:
# grub2-mkconfig -o /boot/grub2/grub.cfg Alternatively, on UEFI systems, run the following: # grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
See the link at https://access.redhat.com/solutions/8709