Reliability is not the main reason people buy SmartThings. (Versatility is.) And if you want to use it for security, really only the ADT/SmartThings Model line is suitable for that, Which is why all the links for “security” on the website take you to that model line.
The following might help ( this is a clickable link):
As for why you saw disconnects on that particular day, that can be very hard to troubleshoot. Have you tried contacting support ?