Can't use a second screen in my Thinkpad P1 Gen3

I just installed Ubuntu 20.04 with the proprietary drivers on my new Thinkpad P1 Gen3 and I can not use the second screen. Ubuntu doesn't recognize it. It is strange, because when I was trying the LiveUSB, I could use both of them.

I've been googling to fix the problem, but the only similar issue was in this post: External monitor no signal ThinkPad P1, with no useful answers. The difference is that in my case, xrandr's output doesn't tell me there is something disconnected.

I tried reinstalling Ubuntu, because I thought it was an error in the installation, since with the LiveUSB both screens worked. I also changed to discrete graphics in the BIOS, but still nothing on the second screen.

Here there are some screenshots:

• xrandr with hybrid graphics:

  

• xrandr with discrete graphics:

  

• Display settings with no multi-monitor options:

  

I tried reinstalling the NVIDIA drivers and realized that the NVIDIA driver is not installed by default, even if you choose the installation with proprietary software. So I installed it, following this guide, restarted, and nothing. The second screen was still off.

Then, I opened the terminal (Ctrl+Alt+T) and, in order to update de system repository, I ran:

sudo apt update

After updating was completed, I ran:

sudo apt upgrade

Then I went to the software manager and actualized everything, included an embedded system Lenovo controller that restarted the PC and made something in the BIOS.

My computer fans started making a lot of noise and after, say 5 minutes, restarting over and over again, I SAW THE LIGHT on my second screen.

So I don't understand exactly what happened, but it worked. I hope this can be useful.