You're falling into the common trap of misunderstanding T-Mobile's "2G speed" data roaming. It doesn't mean "2G GSM", rather it means, "Speed about like 2G would deliver on a good day." It doesn't matter if your connection is 2G, 3G, 3.5G or 4G, your speed will be throttled to about 128 kbps.
Go back into the settings on your phone and reset the connection type to automatic!
This is what I was thinking but was afraid it would result in additional
roaming fees if left to automatic.
No additional charges for roaming 3G/4G.
Japan never used 2G GSM. They had their own peculiar national systems until 3G at 2100 MHz (band 1). They now have 3G and LTE which are compatible for international roaming but no 2G GSM.