Bullet Movement 2D

I'm struggling to make my bullet prefab move forward at a constant velocity when spawned. This is for a top down 2D shooter. Using AddForce doesn't have the desired effect. Here is my code:

PlayerShooting

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerShooting : MonoBehaviour
 {
     public Transform firePoint;
     public GameObject shellPrefab;
 
     public float shotDelay = 2f; //Delay bewteen shots
 
     private float timestamp;
 
     void Update()
     {
         if (Time.time >= timestamp && (Input.GetButtonDown("Fire1")))
         {
             Shoot();
             timestamp = Time.time + shotDelay; //Shot delay is used here
         }
     }
 
     void Shoot()
     {
         GameObject shell = Instantiate(shellPrefab, firePoint.position, firePoint.rotation);
         Debug.Log("Shooting");
     }
 }

PlayerBullet

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerBullet : MonoBehaviour
 {
     public float bulletSpeed;
     private Rigidbody2D rb;
 
     void Start()
     {
         rb = GetComponent<Rigidbody2D>();
     }
 
     void FixedUpdate()
     {
         rb.velocity = new Vector2(bulletSpeed, bulletSpeed);
     }
 }

When spawned, the bullet doesn't move at all. I've been struggling with this problem for weeks. I'm self taught, so please bear with me.