I've installed Windows 10 with Secure Boot enabled, then I've installed Ubuntu 18.04.5 without Secure Boot, the menu was not displayed, I thought it was because of the Secure Boot, but...
My notebook (Dell Inspiron 5537) has a battery problem and displays a message asking to press F1 key to continue or F2 for settings.
I removed the battery and the grub graphical menu was displayed without a problem.
With the battery connected I had to use the `GRUB_TERMINAL=console` option to show the menu.
I have the same problem.
I've installed Windows 10 with Secure Boot enabled, then I've installed Ubuntu 18.04.5 without Secure Boot, the menu was not displayed, I thought it was because of the Secure Boot, but...
My notebook (Dell Inspiron 5537) has a battery problem and displays a message asking to press F1 key to continue or F2 for settings.
I removed the battery and the grub graphical menu was displayed without a problem.
With the battery connected I had to use the `GRUB_TERMINAL= console` option to show the menu.