Flutter app data pass in 2 device without Internet

I want to develop an app where 2 user can pass message without internet connection . Like we can say Diester Management.

The idea came from Flood occurred in my hometown , 2022 . No electricity no mobile network works .

I am new learning flutter , I have no Idea . I see this packages nearby_connections

I humbly request to guys to help me how do I cover come . Thank you .