ModelLimit.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ModelLimit : MonoBehaviour
{
    Transform model;

    // Start is called before the first frame update
    void Start()
    {
        model = transform.GetChild(0);
    }

    // Update is called once per frame
    void Update()
    {
        Vector3 dir = Camera.main.transform.position - transform.position;
        Quaternion toCam = Quaternion.LookRotation(dir, transform.up);
        Quaternion inverse = Quaternion.Inverse(toCam);
        Vector3 rotation = inverse.eulerAngles;
        rotation /= 30f;
        rotation = new Vector3(Mathf.Round(rotation.x) * 30f, Mathf.Round(rotation.y), Mathf.Round(rotation.z) * 30f);

        model.rotation = toCam * Quaternion.Euler(rotation);
    }
}