Chromebook 5190 won't enter developer mode

Hi,

I bought a Chromebook 5190 for my son and I have run into battery issues - he didn't charge it for a while and now I get the flashing amber battery error light.

I found this guide and tried to follow it:

https://www.dell.com/support/kbdoc/en-uk/000130389/dell-chromebook-5190-and-5190-2-in-1-no-power

However the device will not go into developer mode in order to do the firmware update. On the recovery screen Ctrl-D does nothing, and inserting recovery media doesn't do anything either - TAB does work but that's it.

Any advice on how I can recover the device?

Thanks,