I can´t jump while moving automatically

Hello,
Im a bit new to programming and i have a little problem with my code and game mechanic.

What i am trying to do is:
I want to have my Character run constantly without having any acceleration.
I tried many Options, like the Constant Force 2D Component, but in every option there was it!

The Acceleration!

Right now i have an option that works with velocity and a Vector2.
But when i press space for jumping the character goes a little bit up then it resets instantly.

If someone has an Idea, it would be very great.

public class PlayerControllerAutoRun : MonoBehaviour {
 
public float jumpForce = 550;
private bool isRunning = true;
private bool jump = false;
private Rigidbody2D rb2d;
 
void Start()
    {
        rb2d = GetComponent<Rigidbody2D>()
    }
 
void Update()
    {
 
      if (Input.GetButtonDown("Jump"))
            jump = true;
 
        if (isRunning)
        {
            rb2d.velocity = new Vector2(2, 0);
        }
 
    private void FixedUpdate()
    {
        float ver = Input.GetAxis("Vertical");
 
        if (jump)
        {
            rb2d.AddForce(new Vector2(0, jumpForce));
            jump = false;
        }
    }
}

thats because you are resetting upwards velocity back to zero everyframe you are running. when you are running it should only affect forward motion

		if (isRunning)
		{
			rb2d.velocity = new Vector2(2, rb2d.velocity.y);
		}