Adopting the information given within this thread MX-21 Secure Boot to your situation I'd see those steps:
* Boot with MX LiveUSB, which may now boot in secure-boot "disabled in shim" with either standard or also non-signed AHS kernel in the LiveUSB session.
* Open Terminal and run:
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sudo chroot-rescue-scan
=> select the isntalled system to "visit" with chroot
Do now this at the "chroot>" prompt:
Note: No "sudo" needed.
* check to verfy you can see nvram/efivars:
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efibootmgr -v
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lsblk -f
Do this:
* refresh package list
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apt update
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apt install grub-efi-amd64-signed shim-signed
grub-Install twice signed efi-grub-loader and grub:
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grub-install --bootloader-id=MX21 --force-extra-removable
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grub-install --bootloader-id=debian --force-extra-removable
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update-grub
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efibootmgr -v
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exit
Shutdown, remove/pull-out LiveUSB, and reboot.