Fon presents its self as a standard captive portal with I think, a force disconnect every 2 hours, as you are finding. This is purely a function of the CP and you will have to re enter your account details on the FON or BT login screen. There is not really any way to easily automate this without intimate details of the FON CP.
Fon do however do their own Android and iOS app so if you run this on your phone it will reconnect for everyone on your shared network.
