I did not and do not know if you can just substitute a band if the (M/G)Hz is the same. I thought you could not but I’m not sure.
Please do remember the available frequencies don’t say everything. It also matters how good the coverage is, and where you (mostly) reside. For example, for me it is important I have coverage during my commute. In my previous commute I always had a part with no reception, and even though my provider nowadays has lower coverage than competitors (they used to be the best but we are still talking >90% of country) other providers have the same issue.
It could mean, that with your current provider (NTT), for example, they provide 2 high bandwidth frequencies which work very well for your use case. If you can afford the effort involved (time to test, possible effort to resell, money loss if fails) I recommend you try with your current provider (NTT) to see if it is suffice (you’d have band 1 and 3, so you don’t have the lower speed but higher coverage 700/800/900 bands), and if that does not work try Au indeed (as you’d have a lower frequency provided band 18 can use band 26).
Either way, thanks for going forth, and do report your findings it is a way to contribute to the community be it other people in Japan or travelers or people considering going on a business trip etc etc.