Hi, I have not used grml, so I can not comment on that, but I have managed to boot ISOs from disks and network with “pure” grub before.
Make sure that you have secureboot disabled (I guess it already is), and that that there is loopback option in the grub menu entry, and that iso name does not contain any spaces.
Also, if the ISO image is in the UDF format, you need to do insmod udf in grub before setting other options.
Here is one article with detailed explanation that could help you https://www.linuxbabe.com/desktop-linux/boot-from-iso-files-using-grub2-boot-loader