Can't access to smartphone camera

Hi everyone,

I'm trying to build a very simple AR app. Actually, it's like the basic example: you point at a marker and a 3D model pops up. I've been this for ages, also with my old smartphone (Asus Zenfone 2). Now I'm trying with the One Plus 6, and when I run the app on the smartphone I can only see the Unity default "world". I've configured everything, since the app works with the webcam of my laptop.

How can I resolve?

I've attached the screenshot of what i get when I run the app on the smartphone

EDIT: I've tried with the core samples (astronauts), and it works. What could it be?
EDIT": I've tried with my marker and my 3D model on a 2017 version of Unity installed on another machine and it works. I'm hoping to resolve this on Unity 2018 since I don't want to perform a full reinstallation of an older version!