Best phone plan to use while in Japan, already have (US) phone?

I already own a Straight Talk operated
iPhone that I planned to bring with me on my month-long trip to Japan. I know that I'm going to rely on it for directions, Google, calling, etc.HOWEVER, Straight Talk s international service plan does NOT include Japan.

That s a problem.

I have done a little bit of research on mobile Wi-Fi hotspot devices, international SIM cards, etc, but would appreciate advice with this dilemma.

My iPhone says it is out of storage, but it's not?

My iPhone keeps notifying me that I'm out of storage, sometimes it won't even let me take pictures. But when I plug my phone into my PC, it says I have 3gb available on my phone! I've deleted most of my apps and pictures, and still when I view my storage through my phone it says I have zero kb available.

Can't get Cydia working on my phone?

I have a iPhone 6s Plus with iOS 9.3.3 and I jailbroke it with Pangu Semi Untethered Jailbreak. I lost my phone and recently found it. Now I can't open up the app to re install the jailbreak and I accidentally deleted it. Now whenever I download the jailbreak for the no computer version it keeps saying not working retry. Please help me with either re downloading Pangu or remove Cydia.

IPhone SE Help? Please Look?

I'm looking into buying an IPhone SE. I want it only for Wi-Fi and I don't want a carrier. I read that you need a sim card for unlocked phones. I was wondering how this worked, as I've never had one before. Any answers are appreciated. Tysm.