debugging grub2

[daggs]

Greetings,

I have a setup that runs centos installer (CentOS-7-x86_64-DVD-1804.iso) from 
cd in uefi, thing is, when it get to grub, instead of showing the menu, I get 
grub rescue.
ls shows (cd0) but no partitions.
I've verified the installer is ok on another system so this is not an installer 
issue.
I want to enable debugging in grub to see if I can understand what is the issue.
I've mounted the iso and entersed grub.cfg, there I've added this fourlines:
set pager=1
set debug=all
serial --unit=0
terminal_output serial

after the first line (set default=1)
but I still don't get any debug prints in the serial.

how can I enable debug prints?

Thanks,

Dagg.