My understanding of Qualcomm's frequency band support is the same as yours - 6 bands only.
So I can see where you are coming from when the total number of bands supported by the modem is 7 - so which one is missing??

BTW the 7 are:

What is needed is access to te service menu to find which frequencies are available or more specifically which one is dormant.

To date I have been unable to access this data even though the Service Code is *#0011#
This will give you the LTE band currently in use, but not a list of others available.

My suggestion to use a MiFi unit in Japan is to get around the difficulties that you are facing.

I am sure as a regular visitor to Japan you are aware of other options, but in case ou haven't seen this site, here is the link: