Docomo LTE on iPhone 6+ worked about about 6 days around Tokyo and Hokkaido. Then for unknown to me reasons, it fell back onto 3G, periodically lost service, and refused to connect to data when it did have a signal for my remaining 4 days. Enable LTE and data roaming were turned on.

I did select Docomo as the carrier manually and also tried Softbank after the issues started, but Softbank gave "Could not connect to network" error. Behavior was select the network manually, reboot phone, get softbank/docomo 3G signal, maybe get lucky with a bit of data when trying a google search in safari, next time tried get told "Could not connect to mobile data network" on next safari browser action.

A few more things I could have tried:
1. Turn off LTE cellular data to see if 3G would stay latched, but I suspect this might not help as it was falling back to 3G
2. Reset network settings
3. Swapped sim into my android phablet

I suspect the IMEI was recorded by Docomo network and after several days an automated system puts on an LTE blacklist.