Angular2 - NgFor with multiple chekbox

I´m trying to create a table that has multiple checkboxes with different states (some are true, other false).

In this table i have 'Save' button to featch all elements with his state.

The code is like:

<form #form="ngForm" (ngSubmit)="save(form)">
                <table datatable [dtOptions]="dtOptions" [dtTrigger]="dtTrigger"
                  class="table table-hover table-bordered table-striped">
                  <thead>
                    <tr>
                      <th>name-placeholder</th>
                      <th>action</th>
                    </tr>
                  </thead>
                  <tbody>
                    <tr *ngFor="let card of cardsDashboard;let i=index">
                      <td></td>
                      <td>
                        <div class="form-check">
                          <label>
                            <input type="checkbox" name="checkDashboard" [(ngModel)]="card[i]"
                              (ngModelChange)="card[i].activate==true?'true':'false'" [checked]="card.activate">
                            <span class="label-text"></span>
                          </label>
                        </div>
                    </tr>
                  </tbody>
                </table>
                <div class="btn-toolbar pull-right ">
                  <div class="btn-group">
                    <button class="btn btn-primary">save</button>
                  </div>
                </div>
              </form>

The first problem: 'Save' button doesn´t update the state of elements.

Second problem: The checkboxes doesn´t show default state. Checkbox is null.

So the question is: How can i do a ngfor with checkboxes and show the state and update it?