Minimap using RenderTexture with ARCore,Adding minimap to Arcore pointing at a plan with image

I have a Arcore device and I want to create a minimap with a secondary camera pointing at a plane with image with a cylender. it is based on Hello AR. I created a render texture but the image is always grey, any ideas?

,Hi, I have ARCore device with first person camera and secondary camera pointing vertically on a plan with image to show a minimap using Render texture, I am receiving a grey image all the time. any idea if this works? attached an image for the project based on Hello AR.