Before you go spending money, think this through.
Does your iphone satnav work when there is a phone signal?
Does it not work without a phone signal?
If the answer to both is yes, then there is nothing wrong with the GPS receiver. The problem, I suspect, is that the phone uses online maps, which it downloads in small areas as you drive around. Lose the mobile signal and it soon runs out of map information. Typically, the designers don't understand that even an iThing owner might just go somewhere there is no signal.
If I am right (and I don't actually have an iThing), then you cannot buy extra bits to solve the problem.
You may be able to download navigation software with maps to store on your phone - look in the app store.
Or buy a dedicated satnav, or a mobile that stores the maps locally (Nokia for instance). Hidden advantage to this is that if you get it on a different phone provider you will have enhanced security of being able to make a call.
Try here
http://www.carbuyer.co.uk/tips-and-advice/112588/best-sat-nav-apps-2014 for more info on options.