How to reinstall grub2 after installing Windows 11 (on a dual boot system with Ubuntu 18.04)

boot partitioning dual-boot grub2 uefi

I've used ubuntu 21.04 in "try" mode and I have installed Boot-Repair. I've chosen to reinstall grub2 on the /dev/sdb and it did all the necessary steps correctly. After rebooting I was able to boot ubuntu 18.04 installed on /dev/sdb4

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