First thing, thanks for answering so fast :D
So, I've checked those links, but this is a problem after all that. I'll try to explain it in a more easy way:
App1 -> (Main activity/Launcher) UnityPlayerNativeActivity: Shows an Unity logo and the camera starts perfectly, so I can point at my AR images and it shows my 3D model
App2 -> (Main activity/Launcher) Other class with a button* -> press the button -> goto UnityPlayerNativeActivity: shows the unity logo but the camara doesn't start and the screen remains black.
*To open the other class, I use:
Intent intent = new Intent(this,UnityPlayerNativeActivity.class);
Apparently this should work, but it doesn't. Could it be a problem of my phone? I haven't tested in other phones yet, but I'll soon try. If this works in an other phone, I'll leave a reply here, else, I'll be still waiting for some help (if possible)