How to update Unity3D texture from C++ on an Android device?

I want to update a Texture2D from C++ code on an Android device. I pass the texture's native texture ID resulting from the Unity3D call texture.GetNativeTextureID() to C++. In C++ I create an array (unsigned char* img), and pass this to OpenGL functions:

 glBindTexture(GL_TEXTURE_2D, textureID);
 glTexImage2D(
     GL_TEXTURE_2D, 
     0, 
     GL_RGBA,  
     width, 
     height,
     0,
     GL_RGBA, 
     GL_UNSIGNED_BYTE, 
     img
 );

I got no errors (checked with glGetError()), but the array's content does not appear on the texture. What am I doing wrong?