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In Italy there are only few operators that provides a good and reliable coverage everywhere. None of them has an unlimited Gb offer for mobile internet. There are some plan with 30 Gb per month or even more with subscription. But if you stay only some months it would be difficult to subscribe for a long-term engagement.
Anyway, I am not so sure that you can easily require a sim card without having your actual residence in Italy. Once you live here and you get a certificate and all related documents so you can easily go to a shop and buy a sim card in a couple of minutes.