You really need to add sync between devices – Threema. The Secure Messenger Review

This is almost a deal breaker, and certainly keeps from being able to use it for anything but messages that need to be secure. It’s one thing to deal with it for just your account, but when you tell that to other contacts and family it becomes impossible as most people just expect sync no a day's on iOS.
Review by Waswhatwer on Threema. The Secure Messenger.