https://webtrickz.com/how-to-enableturn-on-backlit-keyboard-on-dell-laptops/ ?
It might help to tell us what brand and model of keyboard, what model of Dell laptop.
If you have a gaming keyboard with enough lights for an airport runway, your laptop may not provide enough power to the USB port for continuous use (especially if running laptop on battery). If that's the case, you might try running the keyboard through a powered USB hub to the laptop. Or at least be sure the laptop charger is plugged in while testing.
Also, if you have a gaming keyboard, you may need special drivers for Linux support. But the fact that the backlight flashes on while you are booting is a good sign.