How do I change the display screen to an external monitor?

I have an IBM ThinkPad X4 UltraBase laptop. It's elderly, but useful, works fine.

It has a video port on the back. The BIOS Video settings are: Internal / PCI. I have changed them to no apparent effect.

I have looked, although I'm not sure what I'm looking for, and could find only 'System Preferences Display' and I couldn't see anything relevant.