That’s not a fiat locking wheel nut, so I doubt the fiat dealer would be much help. It’s some sort of after market set.
Search the boot for a locking wheel nut key, if you can’t find one you’ll either have to pay someone like quick fit to remove it or buy a set of “bolt removal” sockets, and do it yourself, can find them on amazon for as little as £18 and then you’ve got them forever, just search for locking wheel nut removal tool on amazon, the results will actually be but removal tools which will also remove rounded off nuts and bolts as well as wheel nuts.