We are working in a project using buses with the following issue:
In a typical travel across Mexico the busses just have cellular coverage about 50% of the time; so it´s necessary that the customer can see the Captive Portal in their mobile devices and then should be redirected to a local server into the bus (For example: 192.168.50.20:81) even without cellular coverage.
We have made some tests and we couldn´t display the Captive Portal without internet connection.