M4800 Boot problem

Hi everyone! I just bought used M4800 with Windows 7 installed (worked fine) and immediately decided to change it to Windows 10 and started having big issues. This is how it went:

1. After installing W10 and rebooting I got this messege shown: "Invalid partition table". Quick Google showed me that it might be outdated BIOS version (I think it was A13 or A15 installed). So I downloaded latest BIOS (A25) from Dell website and proceeded with update.
2. I did every step from this article: https://www.dell.com/support/article/us/en/19/sln305230/flashing-the-bios-from-the-f12-one-time-boot-menu?lang=en Message showed that update was complete and laptop rebooted itself. 

3. But after reboot the screen stayed turned off, nothing was happening, didn't see DELL logo. Only LEDs from power button, volume control and ssd activity was on. I could turn off the laptop with one push of power button.

4. I inplugged the CMOS battery for 15 minutes, plugged it back on again and turned on the laptop. DELL logo appeared, I could go into BIOS settings. Stayed there for few minutes and after reboot screen was black again (and this is happening every time I use laptop more than few minutes and try to reboot) By trial and error I discovered that if I leave the laptop for about 10 minutes and then power it on I can see DELL logo and boot into Windows, BUT laptop screen stays off. I can only use laptop on external monitor.

Additional notes: tried installing Ubuntu, same behavior as with Windows. When I try live usb laptop screen works just fine, sometimes in BIOS and other pre-windows menus keyboard stops working and to do anything I need to reboot. Don't know if this keyboard problem is present in Windows. 

Does anyone hava a clue what is happening to this laptop?