Installing Ubuntu with secure boot on and no way to disable it?

I don't have any OS installed. I wanted to install Win10 but it failed with error code 0x80070570 during install.

Now, I want to go back to Ubuntu but I am not able to (Only Unetbootin screen shows with a 'Default' option, and no way to move forward). When I boot the computer I am not able to go to BIOS or Boot options even if under Dell logo it says F2 for Setup and F12 for boot options. I think Win10 enabled secure boot or fast boot (?!).

I am stuck! I can't go to UEFI firmware settings and I cannot boot Ubuntu USB stick. What can I do?

Is there a command that I can use in order to access UEFI firware settings next time I boot (I have a Win10 setup USB)?

Some computers will bypass an attempt at entering BIOS/UEFI after a restart, but will enter it just fine from a full power down. Some will only register the F2/F12 if pressed a just the right time.

Power the laptop down completely (hold the power button until all lights turn off and the fans stop spinning if necessary), then power it on again, then quickly and repeatedly press F2 until the Dell logo goes away. This should get you back into BIOS/UEFI and allow you to disable Secure Boot and/or UEFI boot mode (if necessary).