Can I test the internet connection in Japan before traveling abroad?
No. You cannot test the internet connection in Japan.
When you can use your WiFiBOX
A WiFiBOX rented for use abroad will connect to the internet after you arrive at your destination.
Why it cannot connect in Japan
Your WiFiBOX can only connect in the countries covered by your selected plan.
Therefore, a plan for overseas use cannot connect to the internet while you are in Japan.
Example
If you have booked a plan for South Korea, you will be able to connect to the internet after arriving in South Korea.
Turning on your WiFiBOX in Japan will not allow you to test its internet connection.