You'll be in Japan! Get your face out of your phone and enjoy your trip!
Also, download maps/videos/songs/e-books ahead of time over Wi-Fi, use Wi-Fi where you have the option (hotels, coffee shops, etc.), turn on "low data mode," stream less audio and video (and adjust video quality to 480p or less), use WhatsApp and FaceTime audio instead of video, disable automatic e-mail attachment downloads, turn off automatic video playback in social media apps, don't install apps or operating system updates over mobile data (and turn off automatic updating over mobile data).... In short, learn how to be a data sipper instead of a data glutton.