Official Raspberry Pi 7 inch touchscreen not initialising

I’m using the DietPi image on Rock Pi 4b, booting into chromium in kiosk mode. My official Raspberry Pi 7 inch touchscreen (800x480) is connected to 5V + GND and DSI with the ribbon cable. My kiosk appears on an HDMI monitor when but there’s no activity, no backlight, nothing, on the touchscreen.

I’ve installed the rockpi4-dtbo package and created /boot/hw_intfc.conf with the line intfc:dtoverlay=raspberrypi-7-inch-lcd uncommented. I’ve tried adding overlays=raspberrypi-7-inch-lcd to /boot/armbianEnv.txt but nothing.