SmartThings is not fit for purpose as a security system. 
It’s not just my opinion, the company says so themselves in their product usage guidelines.
It’s very important to understand that smartthings is primarily a cloud-based system. That means that not only does the Internet have to be working, but the smartthings cloud has to be available even just to change the status of the system from armed to disarmed. (Seriously, there is no way locally to change the mode.)
And there is no way, none, to send notifications, even to your own phone, unless the smartthings cloud is available.
And the app won’t work at all unless the smartthings cloud is available even if it is on the same Wi-Fi as your hub.
They didn’t have to design it that way, but they did. At the time of this writing, The only part of smartthings which is even eligible to run locally is the official smartlighting feature, and then only some parts of it and only with some devices. All “custom code“ runs in the cloud. You don’t get to load it onto your own hub.
So even if you were just looking for a selfmonitored notification system, Smartthings is just not suitable for most people. It’s OK if you just want convenience notifications, like a reminder that a guestroom window was left open when rain was expected. But not for a notification that your house is on fire or someone has come in through a window.
There are a number of other security systems in the DIY low cost tier like Ring and Yale that have battery back ups and cellular communications. It’s just that smartthings isn’t one of them. So many of us, myself included, run a separate security system.
See the following. (Again the topic title is a clickable link.)