Nice idea, the router can get the best of both worlds, mobile or land line broadband.
But I don't really see how it can work, unless the router has its own SIM for mobile access, which implies a subscription that would likely be cripplingly expensive compared to any stand alone broadband package.
If you have an iPhone, you can already configure it to act as a WiFi hotspot for other devices, if landline broadband goes awol. I am certain Android will have a similar feature. These cost nothing extra, other than the metered data if usage should exceed allowance.