If youre on TMO , you probably dont need to rent a pocket wifi. I and a bunch of other people I know traveled to Japan without renting a pocket wifi and it worked fine, slow.. but fine. I used it for gmaps, apps for the train/subway and some pokemon go.

As for your iphone being stolen.. wont happen. My cousin left her phone and wallet at a coffee shop and came back an hour later, it was still there. A friend left his wallet at a busy restaurant in one of the train stations, it was still there when we returned. Bikes with shopping bags left outside unlocked everywhere. Its not San Francisco thats for sure.
Japan is an amazing country, do not worry.