OpenWrt on RevolutionPi Core 3

Hello there,

I'm trying to setup OpenWRT on a RevPi Core 3 of KUNBUS but I don't get it up and running. I tried both images for the Raspberry Pi 3 but I'm only seeing the rainbow screen. Setting it up under ubunutu. I connected a keyboard and a display with hdmi. It has enough power with a 24 V power supply.

Looking forward for help