I swapped my SIM card into a new iPhone 6s and now it won't recognise my data so can't access internet without wifi

Hello!

I bought an iPhone 6s yesterday and obviosuly when I was in the Apple Store, I was connected to wifi. When I left I had to turn on my 3G to search for an address to put in my sat nav and I got a message on my screen saying "

Could not activate mobile network. You are not subscribed to a mobile data service" all I did was swap the SIM card from an iPhone 5 to the iPhone 6s. I do have a data package on my vodafone sim.

Hope one you can help!

L x