3D model problem (3DsMax,Unity)

I have build some models in 3DsMax, and the model looks normal inside.
Inside 3DsMax
https://media.discordapp.net/attach...937921105921/unknown.png?width=708&height=517

But after export the model to the Unity, there is some problem on the model.
In some view angel, the surface of the model is disappeared, and when the player walk to the place, the player may stuck in the model.

Inside Unity:
Look from below:
https://media.discordapp.net/attach...161179734056/unknown.png?width=776&height=662
Look from above:
https://media.discordapp.net/attach...291140227072/unknown.png?width=555&height=489
Look from beside:
https://media.discordapp.net/attach...959594848266/unknown.png?width=931&height=483

The problem appear in more than one place.
What should I do to fix the problem? Is there any keywords I can search in Google to fix the problem?
Please help! Thank you!

 

in 3dsmax try adding "Normal" modifier to flip normals.

 

Thank you so much! It worked!