It's a strange question: i'm using a Raspberry Pi3 as a 'router' for mobile devices (Android / iOS). Raspberry will be NEVER connected to internet, so if a mobile devices connect to its WiFi, it will only be able to connect to the raspberry's local sites.
I know that Android / iOS switch automatically to 3G/4G if the WiFi can't reach internet.
In my example, i have a local website on Raspberry that has a link to paypal.com. The problem is that if i click (from my mobile device that is connected to rasp's WiFi) the link to paypal.com, my device tries to get the page using WiFi, but after 30 seconds it goes in timeout. I'd like to 'notify' to devices that the WiFi have no access to Internet, so a smartphone can automatically pass to 3G/4G for 'external' sites.
I'm using Hostapd+DNSMasq. I'd like to have the same behavior of the 'Parrot Drones' (your mobile device gets connected to the drone trough wifi but still uses 3G/4G because the device get notified that there is no internet connection).
How can i configure my raspberry wifi to notify that there is no internet connection?