La pregunta ya ha sido respondida en SO. Aquí está la referencia:

Activador de notificaciones de iOS: quincenal y / o trimestral

Pero lo que he intentado es hasta ahora:

var fortnightPart1 = DateComponents()
fortnightPart1.weekday = Calendar.current.component(.day, from: Sdate) //(Day..here Sunday)
fortnightPart1.weekdayOrdinal = 2 //= n == (nth Day in the month...here 2nd Sunday in every month month)
fortnightPart1.hour = Calendar.current.component(.hour, from: Sdate)
fortnightPart1.minute = Calendar.current.component(.minute, from: Sdate)
fortnightPart1.second = Calendar.current.component(.second, from: Sdate)
trigger = UNCalendarNotificationTrigger(dateMatching: fortnightPart1, repeats: repeate)

No puedo lograrlo después de 14 días. Aunque, se trata de datos aleatorios. ¿Alguien puede verificar qué tiene de malo el código?

Actualizar:

Según su sugerencia (https://stackoverflow.com/a/46072497/7408802), aquí está el código final que funcionó!

//for 1st time notification
 let center = UNUserNotificationCenter.current()
 let content = UNMutableNotificationContent()
 content.title = title
 content.body = body
 content.categoryIdentifier = "\(id)"
 content.sound = UNNotificationSound.default()
 fortnightPart1.hour = Calendar.current.component(.hour, from: dateToFireON)
 fortnightPart1.minute = Calendar.current.component(.minute, from: dateToFireON)
 fortnightPart1.second = Calendar.current.component(.second, from: dateToFireON)
 trigger = UNCalendarNotificationTrigger(dateMatching: fortnightPart1, repeats: false)
 content.userInfo = ["NotificationID": id,"repeate":repeate,"resedule":true]
 let request = UNNotificationRequest(identifier: "\(id)", content: content, trigger: trigger)
 center.add(request)

//when first notification triggered, we need to set another for 14 days repeat in UIApplicationDelegate
func application(_ application: UIApplication, didReceive notification: UILocalNotification) {
    let center = UNUserNotificationCenter.current()
    let content = UNMutableNotificationContent()
    content.title = notification.alertTitle!
    content.body = notification.alertBody!
    content.categoryIdentifier = "\(reminderId)"
    content.userInfo = ["NotificationID": reminderId,"repeate":true,"resedule":false]
    content.sound = UNNotificationSound.default()
    let trigger = UNTimeIntervalNotificationTrigger.init(timeInterval: 1209600, repeats: true)
    let request = UNNotificationRequest(identifier:"\(reminderId)", content: content, trigger: trigger)
    center.add(request, withCompletionHandler: nil)
}