Dual Boot: GRUB disappeared, boot-repair not working and linux partition cannot be accessed
I think you are trying to fix the wrong issue.
You can get a bootable Ubuntu with Secure Boot enabled even if it is not detected (caused by non-standard UEFI firmware behavior) or you somehow lost the EFI binaries on the EFI system partition.
Have a look at: How to use the Linux Foundation's PreLoader?
Sadly almost no one cares about getting Secure Boot to work properly. If boot-repair fails it has to be an evil Microsoft, the reptilians or some other conspiracy.