How to Get Value of a Formcontrol in Angular4

I have some experience in Angular4, but I just inherited a piece of code that uses FormControls, and I don't know how to work with them. I am setting up a comments textarea that is required if the value of isRegulatoryAuditRequired equals false. I've got the required field working, but I need to display the message 'Comments required' if value is false and the form is submitted. Here is my HTML, with two radio buttons, a textarea and a label for the message:

<div class="btnWrapper">
  <div class="leftBtn">
    <span class="col-xs-12 no-padding">
      <label>
        <input type="radio" id="radio1" class="app-input-radio" name="isRegulatoryAuditRequired" [value]="true"
               [checked]="isRegulatoryAuditRequired==true" formControlName="isRegulatoryAuditRequired" />
        <span class="app-input-radio pull-left"></span>
        <span class="plx5 pull-left radioText ">Yes</span>
      </label>
    </span>
  </div>
  <br />
  <div class="rightBtn">
    <span class="col-xs-12 no-padding">
      <label>
        <input type="radio" id="radio2" class="app-input-radio" name="isRegulatoryAuditRequired" [value]="false"
               [checked]="isRegulatoryAuditRequired==false" formControlName="isRegulatoryAuditRequired" />
        <span class="app-input-radio pull-left"></span>
        <span class="plx5 pull-left radioText ">No</span>
      </label>
    </span>
  </div>
</div>
<div class="row mbx20">
  <div class="col-sm-4">
    <div>
      <label>Why is the regulatory audit being performed or provide additional comment?</label>
      <div>
        <textarea id="comments" name="Text1" [required]="isRegulatoryAuditRequired==false" cols="100" rows="2" formControlName="comments"></textarea>
        <label *ngIf="commentsRequired('comments')" style="color:red;font-weight:bold">Comments required</label>
      </div>
    </div>
  </div>
</div>

I need to evaluate the value of the 'isRegulatoryAuditRequired' and 'comments' FormControls to see if I should display the 'Comments required' message. Here is the method I'm trying to use, but it isn't working:

commentsRequired(controlName: string) {
  const control = (<FormControl>this.rotationForm.controls[controlName]);
  const isRegulatoryAuditRequired = (<FormControl>this.rotationForm.controls['isRegulatoryAuditRequired']);
  if (isRegulatoryAuditRequired.value === false && control.value === '' && this.formSubmitAttempt) {
    return true;
  } else {
    return false;
  }
}

The value of 'isRegulatoryAuditRequired.value' and 'control.value' print to the console as null, which therefore doesn't display the message. How do I retrieve these values? Any help will be appreciated.