3D model problem, model only showed one side!!!

https://media.discordapp.net/attachments/867396579728162841/937213458422853632/2022-01-30_1.11.01.png?width=911&height=601 Photo1

https://cdn.discordapp.com/attachments/867396579728162841/937213458783543316/2022-01-30_1.11.20.png Photo2

https://cdn.discordapp.com/attachments/867396579728162841/937213458997444638/2022-01-30_1.11.27.png Photo3

What should I do? The model always have some side missing. It looks normal in 3ds max, but when it moves to unity, some of the model will have the problem. And when player walks to the model, the player will stuck inside.

Thank you! And sorry for my poor English.