Use this command to show the GRUB menu
sudo sed -i 's/TIMEOUT_STYLE=hidden/TIMEOUT_STYLE=menu/g' /etc/default/grub
Then this command to uncomment the option for os-prober to find the proprietary OS
sudo sed -i 's/#GRUB_DISABLE_OS_PROBER=false/GRUB_DISABLE_OS_PROBER=false/g' /etc/default/grub
then update grub and reboot
sudo update-grub && reboot