Implementing a state pattern

I have an object that based on it's properties, will perform different operations in it's service layer.

Let's say it's a customer with the following states: Registered, RegisteredWithoutContactOnFile, and NoRegistration.

These states are based on properties set on the object. So it would be something like the following:

public class Customer
{
        public Registration Registration {get; set;}
        public string Email {get; set;}
        public int Id {get; set;}
}

Then there would be the CustomerService which does different database operations based on what is entered like such:

public class CustomerService
{
        public void RegisterCustomer(Customer customer)
        {
               if (customer.Registration != null && customer.Id == 0) //Registered without contact

               else if (customer.Registration == null && customer.Id == 0) // No registration/contact
               
               else if (customer.Registration != null && customer.Id > 0) //Registered and contact on file
}

I basically want to get rid of this if/else statement in the service layer and make it a state in my object so that I can more easily test it. Can anyone assist with how I would go about this or if I'm going about this the wrong way? I will say I am interested in the state pattern as I have been working on Finite State Machines for an AI agent but I feel like it makes sense here as well. Perhaps a strategy pattern would work? In short, I'm just trying to get the logic out of the service layer to make it more testable... any insight would be greatly appreciated!