I bought one of these kits just before our last trip down to Spain:- http://www.motorhomewifi.com/product...mount-antenna/
Not the cheapest way of doing it but it all works as advertised and we had excellent reception all through France and into Northern Spain.
I used a 3 sim and stuck the antenna on a side window and just left it all connected to a usb power socket. It gave us the same sort of wi-fi connection as our home router but at twice the speed.
Excellent bit of kit and if you run into problems you can ring Motorhome Wi-Fi for advice.