GRUB2 not showing on HDMI monitor

I have an Acer Aspire E 15 laptop that I use at home most of the time, connected to an HDMI monitor and USB keyboard and mouse. I have Windows, Ubuntu and openSUSE installed on it. I would like to use it with the lid closed, but I always had to open its lid at least for powering it up and selecting the operating system I wanted to use during boot. GRUB2 always showed only on the laptop screen, even if the HDMI monitor was plugged on. There was no way to get GRUB2 displayed on my HDMI monitor.