In the absence of reliable wifi in the area where you are staying in Japan, there are SIM cards available to travellers, both with short-term and long-term data options. SIM card validity starts from 7 days to months. Data speed starts at around 200MB per day. Price varies depending on validity and data speed but it starts around 2000 yen.
One of the most recommended travel data SIM cards in Japan is the Sakura Sim. This brand even offers daily packages for those travelling in a tight budget.