Access grub menu on Ubuntu Core 18.04 (Raspberry Pi 4)
Solution 1:
There are several "wrong assumptions" in your post.
- There is nothing called Ubuntu Core 18.04. There is Ubuntu Core 18, and the latest core release is Ubuntu Core 20.
- The Raspberry Pi images (of either Core or LTS) does not include GRUB at all (it uses u-boot instead).
So what you're asking is actually not possible, since there should be no GRUB in your installation.
I don't actually know if the Raspberry Pi can support GRUB at all.