Basically you can use unlimited 4G data with advance mode of 603HW. However, unlimited settings may not allow you to stay connected if you are using it in places like subway and basement. In addition to this, if your room is surrounded by walls, there is a possibility that radio wave from outside cannot be received well. In this case, you can relocate pocket WiFi to the place like near window. However, it still may not work depend on your surrounding environment.
There is a solution for this. Unfortunately, this solution will not lead you to use unlimited data; however, you can still stay connected under difficult environment. This applies to almost all pocket WiFi in Japan. When a WiFi rental company in Japan says you can use unlimited data, it means you can use unlimited 4G data, not 4G LTE. Some places will say you can use unlimited 4G LTE. In this case, the usage of 4G LTE is limited to 1-3 Gigabytes per day and speed limit will be applied after using all of these.