Apart from edit /boot/grub2/grub.cfg or /etc/grub2.cfg. We’d better use grubby to config grub.

Here is the example of how to select default grub.

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# grubby --default-kernel
/boot/vmlinuz-4.20.0

# grubby --default-index
1

# grubby --set-default-index 0

# grubby --default-index
0

# grubby --default-kernel
/boot/vmlinuz-4.20.0+

Grub missing

If grub missing due to anyreason, and you are fall back to grub menu, you can try

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grub> insmod all_video
grub> insmod gzio
grub> insmod part_msdos
grub> insmod xfs
grub> set root='hd0,msdos1'
grub> linux /vmlinuz-4.20.0 ro rhgb
grub> initrd /initramfs-4.20.0.img
grub> boot

As the kernel version is not the same, you can use TAB to find and complete the name

Reference: Grub menu missing Linux Kernel entries

For RHEL8, the loader entry is under /boot/loader/entries/ and the bootup option is in /boot/grub2/grubenv