I have an HP EliteBook 840 G7 Notebook, with Linux Mint 20.3.
My goal is to connect two more monitors to the machine.
I have a monitor connected via HDMI port, and a monitor connected via USB-C port.
Both monitors at the moment work if connected individually, but if I connect both, the one on HDMI takes over, and no signal goes to the one connected via USB-C.
The strange part is that I have been able to work with three monitors for some time, up until (several months ago) I have upgraded the system (from 20.1 to 20.2, I think) ad I lost this ability.
After reading this answer I have tried to change settings in the bios (allocated bigger video memory, and "enabled high resolution mode" (something involving USB devices in a dock, and Gigabit NIC), but I had no results.