How to have enemy only shoot when they are within a certain range?

I made a few changes to your code, hopefully it works...

Changes start at lines 20, 32 & 61 and all have comments explaining them.

 using UnityEngine;
 using System.Collections;
 using PlayerInfo;
 using Shell;
 
 namespace EnemyInfo
 {
     public class EnemyMovement : MonoBehaviour 
     {
         public Transform player;
         public Transform EnemyTank;
         public int AttackRange = 35;
         public bool inRange;
         public int TooClose = 10;
 
         NavMeshAgent agent;
         static PlayerHealth pHealth;
         static EnemyHealth eHealth;
         
         // change the name of the Rigidbody variable to have a lower case 'r' so that the compiler won't confuse it with the Type Rigidbody
         // (variable type names are reserved words, so you should never use these words with the exact capitalization for a variable name)
         // remember to change the name where you use it below too
         Rigidbody rigidbody; 
 
         void Awake ()
         {
             player = GameObject.FindWithTag ("Player").transform;
             pHealth = player.GetComponent<PlayerHealth> ();
             eHealth = GetComponent<EnemyHealth> ();
             rigidbody = GetComponent<Rigidbody> ();
             
             // EnemyTank is never initialized (unless you did that from the inspector)
             // assuming this script is attached to the EnemyTank, if so then this would work:
             EnemyTank = transform;
         }
 
         void OnEnable ()
         {
             rigidbody.isKinematic = false;
         }
 
         void OnDisable ()
         {
             rigidbody.isKinematic = true;
         }
 
         void Update ()
         {
             NavMeshAgent agent = GetComponent<NavMeshAgent> ();
 
             if (pHealth.CurrentHealth > 0 && eHealth.CurrentHealth > 0) 
             {
                 agent.SetDestination (player.position);
                 transform.LookAt (player.position);
             } 
             else 
             {
                 agent.enabled = false;
             }
 
             // ok, so this part needs to be changed just a little bit
             // firstly we're going to store the Distance in a variable to avoid multiple calls to the exact same function
             float distance = Vector3.Distance (player.position, EnemyTank.position);
             
             // there's no need to use nested if statements here - you could just check both conditions in the same IF
             // BUT there's an even quicker way to do what you want:
             // inRange will be True if the condition is True - otherwise it will be False
             // therefore just do this:
             inRange = ((distance < AttackRange) && (distance > TooClose));
         }
     }
 }

Let me know if this fixes your problem :)