Solved:
<input
name="datepicker"
ngbDatepicker
#datepicker="ngbDatepicker"
[autoClose]="'false'"
(dateSelect)="onDateSelection($event, datepicker)"/>
and
onDateSelection(date: NgbDate, datepicker: NgbInputDatepicker) {
// Close the datepicker if both fromDate and toDate are selected
if (this.fromDate && this.toDate) {
datepicker.close();
}
}