Use this to de-cloak the GRUB menu
sudo sed -i 's/TIMEOUT_STYLE=hidden/TIMEOUT_STYLE=menu/g' /etc/default/grub
If the GRUB menu has been hidden then it is likely that the new setting for os-prober has been added to GRUB configuration but is commented out
# Uncomment this option to enable os-prober execution in the grub-mkconfig command
#GRUB_DISABLE_OS_PROBER=false
To uncomment this option
sudo sed -i 's/#GRUB_DISABLE_OS_PROBER=false/GRUB_DISABLE_OS_PROBER=false/g' /etc/default/grub
then
sudo update-grub && reboot