Help with AR and FBX imported

Hello
Im working with examples with vuforia and all is running ok.
I do a test with the astronaut fbx model and work perfectly.

i have then exported from 3dmax 9 an object and i get this result:

5057801--496631--upload_2019-10-12_0-40-58.png

the object is ok, as i signale it with the red arrow, but i get a floor under the object, in grey color.
Can someone please give me some help in order to remove this?
thanks in advance

Nobody can really help me?