All seems to be going well until the home’s wifi goes down, an awful matter made even worse when a pair of strangers, G.H. (Ali) and his daughter Ruth (Herrold) show up at the front door insisting that the house belongs to them.

Source: https://collider.com/leave-the-world-behind-sneak-peek/

Ruth's alarm goes off at the same time as G.H.'s, which also syncs into what looks to be bombs dropping in New York City, possibly marking the collapse of society in the US after people turned on one another exactly as G.H. explained to Clay.

Source: https://collider.com/leave-the-world-behind-ending-explained/