XPS 17" L702X - BIOS update - now won't boot

Hello BIOS experts,

Hi, I have an XPS 17" L702X which had the A12 BIOS. I updated to the A19 BIOS which involves running a program in Windows which then reboots and flashes the bios. The process was not interrupted.

So the laptop rebooted, the BIOS flashed, and now I am left with a machine that won't boot.

I switch on the power, the fans go on, the power light goes on, the three icons above the keyboard next to the WAVES icon light up sequentially, and almost exactly 10 seconds later, the laptop powers down. There is no display or beep.

I have tried taking out the battery and AC, holding down power, then attaching the AC again, no help.

I think it's a Phoenix BIOS but I'm not 100% sure. This is where I downloaded it from:

http://www.dell.com/support/home/us/en/19/Drivers/DriversDetails?driverId=CPF0J

I tried using the WinCrisis BIOS recovery software described here:

https://www.bios-mods.com/bios-recovery/phoenix-bios-recovery/

however I ran into a couple of issues 1) that the tool didn't seem to change the contents of the USB drive (however it says in the guide "Reconnect the USB flashdrive and copy over MINIDOS.SYS, PHLASH16.EXE, and BIOS.WPH to the root directory." so I don't know what's going on there)

and 2) I don't know what key combination to use to trigger it...

Can anyone help please? Thank you very much in advance.