Docomo iPhone 5S and 5C ARE compatible with MVNOs

However, there is a catch. According to the b-mobile compatibility page, LTE only works with voice SIMs. With a standard data-only SIM, you will only get a 3G signal.

LTE接続は音声SIMの場合にお使いいただけます。

LTE requires use of a voice SIM.

This leaves two options, ruling out most other MVNOs because they only offer data:

• Smartphone LTE SIM (スマホ電話SIM LTE)
• Aeon Smartphone SIM (イオン専用スマホ電話SIM)

In addition, a new carrier profile is needed, which is available from b-mobile. This also means that visitors without residency cannot get an LTE signal with an iPhone. Also, this carrier profile does not allow tethering.

I have absolutely no idea why is the problem with LTE and data SIMs, or what is preventing the phone from being able to tether, even with a different carrier profile.