I have a Teclast F15 with dual boot Ubuntu 20.14/Windows 10. I a, having problems with the HDMI output under Ubuntu.
With Windows the HDMI output can be set to mirror the laptop screen OK. With Ubuntu the ‘Screen Display’ settings can be set up correctly. Whatever I set them to when I click ‘apply’ they revert back and my second screen is blank. I’m sure it was OK with 18.14 as I connected a projector OK.
