@doice: the OP wrote that the chair was exported from C4D (Cinema 4D), so i guess he used this program.
His issue is a result of an incorrect model setup and can be fixed with a few changes like merging objects.
@Miguelillo:
What exactly needs to be done, depends on the model and it’s setup.
For further guidiance i would need to know more about the model and it’s current setup.
So if you can share the file or at least post some screenshots of the model in C4D (wireframe, object manager hierarchy)
i can help you with bringing it ingame as a single mesh.