You need to use Angular template reference variable (for example #sidenav) on the sidenav HTML element. This is example code from our documentation:

<mdb-sidenav #sidenav="mdbSidenav">

Then in TS file use ViewChild decorator to get access to the component and its methods. If you want to use method on component init, it will be available in ngAfterViewInit hook:

@ViewChild('sidenav') sidenav: MdbSidenavComponent;

ngAfterViewInit(): void {
  this.sidenav.show();
}