There are many projects similar with Briar, they all have the same limitations:

"The range of Bluetooth and Wi-Fi is around 10 metres, depending on obstacles. Clearly this isn’t enough for communicating across a city or even a large building. So when Briar receives a message from a nearby contact, it stores the message and can later pass it on to other contacts when they come within range (for example, when you move from one place to another)"