Ubuntu – Dell Latitude 5490 BIOS update broke Ubuntu

so this post https://andidittrich.de/2015/12/ubuntu-15-10-with-dell-wireless-5570-hspa-42mbps-mobile-broadband-card.html talks you through a solution

1) apt-get install libqmi-utils

2) gksudo gedit /etc/udev/rules.d/99-dell5570.rules

.... that assumes gedit is installed as a text editor; you will be creating a new file and you paste all the text in the above post that they recommend into your new file

3) but you must change the ID of the example; to yours;

so where it says ATTR{idProduct}=="81a3"

then you must change each example to ATTR{idProduct}=="81b6" ..cos that is yours ..

; SAVE; CLOSE; REBOOT ........... any joy?

Am I doing the wrong thing?

Yes, I think so. There is a page dedicated to BIOS updates and it says about Dell this:

If you are using UEFI and your F12 boot options include "Flash BIOS upgrade", one may download the BIOS upgrade .exe from Dell's website, and put it to your /boot/EFI/ folder. Reboot and select "Flash BIOS upgrade" option. In the dialog select the .exe file you have just downloaded and continue with the process.

For more on Dell specific procedures, please see here.