A relative told me about this page as I'll be travelling to Japan soon on the Jetstar return for free deal (thanks OzB!).
There are two options:
1. Trip Free 500 MB, 7 Days
You must register at least 10 days before departure. You'll get 500 MB for free, and can earn more data by filling out a survey (300 MB), book restaurants or activities (100 MB). SIM cards can be picked up at the airport or delivered to your hotel. Note that the SIM must be returned in an envelope prior to returning or get charged the deposit (Japan has this law restricting foreigners from purchasing SIM cards so a workaround is for telcos to "rent" SIM cards out. Data-only SIMs are exempt from this restriction). More data can be purchased.
2. WAmazing 500 MB, 15 days
Site and app are in Mandarin so you need some OzB April Fools skills. Register before departure — it didn't say the minimum number of days before you leave. SIM can be picked up at the airport. You must download the app (Taiwan/HK/China Apple Store) and register through there. Again, more data can be purchased. Share on Facebook through the app to get another 500 MB. Didn't say anything about returning the SIM.
Apparently you get unlimited data if you book your hotel through the app. Haven't tried it yet.
Neither says anything about speeds, but I assume it'll be 3G/4G.
ENJOY!
Source: https://choyce.tw/naritavsc/ (in Mandarin)
OMG. This comes on time. We depart in 12 days. Thanks.