I have the same notebook, and my only complaint is that quite often the notebook monitor doesn't switch on until I connect or disconnect an external HDMI monitor. A pity.
I don't have that problem - it works flawlessly all the time. I am using Amazon basics HDMI cable connected via HDMI switcher to the external display (with the desktop, Chromecast and Tronsmart Android stick occupying 3 more HDMI inputs). Try perhaps another HDMI cable?
I think that's something with my machine, but never wanted to wait for a repair. BTW the HDMI is not a problem: The laptop monitor is often not working when I boot the pc up without any external monitor. But then, attaching one, it will start working.
