just restating what has to be done:
either do A
followed by C - as is always necessary when /etc/default/grub is changed and you want the change to take effect
or
do B
(create and edit the file /etc/sysctl.d/99-sysctl.conf
)
D - whichever option of the two above you chose
reboot afterwards to have the changes in effect
You don’t have to reboot if you do this as well:
echo "1" > /proc/sys/kernel/sysrq
as root
This gives you the functionality immediately - but it “doesn’t stick”, doesn’t survive a reboot.
That’s what either A+C or B will do …
it’s a three key combination each time - for each individual letter
(for me - but you can test, so you know the procedure once you need it)
Each key combination does a part of the process to relatively safely reboot an unresponsive system.