How to enter rescue mode (RescueInitramfs) on Debian server?

I am trying to boot into RescueInitRamfs (https://wiki.debian.org/RescueInitramfs) on my Debian server after configuring DropBear as per https://benediktkr.github.io/ops/2015/05/01/remote-fde.html (mostly). I have tried updating the kernel boot parameter in file /etc/default/grub by setting GRUB_CMDLINE_LINUX_DEFAULT="quiet rescue/enable=true" and GRUB_CMDLINE_LINUX="rescue/enable=true" (as per https://www.debian.org/releases/stable/amd64/ch08s06.en.html) , then running sudo update-grub. I have verified that the kernel is being booted with this parameter by inspecting /proc/cmdline. It seems that this kernel parameter has no effect and the system is booting straight into "normal" (not rescue) mode. I then tried to boot into rescue mode by setting the kernel parameter systemd.unit=rescue.target and now I am unable to SSH in but the server seems to be online as I can ping it. 1) Why is rescue/enable=true doing nothing? 2) What is the difference between rescue/enable=true and systemd.unit=rescue.target? 3) What should I be doing to get this working with DropBear? UPDATE - to clarify, I only have SSH access. No console.