If you do not want to use jQuery, make the button type a normal button and add a click listener pointing to the function you would like to execute, and send the form in as a parameter. The button would be as follows:
<button (click)="yourSubmitFunction(yourForm)" [disabled]="!yourForm.valid" type="button" class="btn btn-primary" data-dismiss="modal">Save changes</button>
Remember to remove: (ngSubmit)="yourSubmitFunction(yourForm)"
from the form div if you use this method.