I believe that's because there US S7 doesn't have the LTE receiver for the data band used in Japan, whereas the 5X does and not the issue of the Fi SIM. I bet i

I believe that's because there US S7 doesn't have the LTE receiver for the data band used in Japan, whereas the 5X does and not the issue of the Fi SIM. I bet if you got an S7 from Japan it would have worked fine