It perfectly worked! WiFi Tether also worked perfectly, which is amazing!

1. ✔ Internet Connections: Safari, Gmail App and so on.
2. ✔✔ Phone Call
   • ✔Receive
   • ✔making phone call
3. ✔✔ SMS
   • ✔ receive
   • ✔ send
4. ✔ WiFi Tether

Good Reference: Shu's Blog (in Japanese)

1. Turn off "Find iPhone" on your iPhone.
Note: without doing this, you can not intall .ipcc. (See below).
1. Settings -> "Apple ID, iCloud..." -> Find iPhone.
2. Settings for APN.
1. Download the setting file: apn-profile.mobileconfig or
2. Download the setting file2: rakuten-v6.mobileconfig
3. Import the file: Settings -> general -> Profile -> import the file
3. Command Prompt: set Carrier Testing mode on in iTunes.
Do one time is enough, even if you have more than 1 iPhones.
1. Run Command prompt on windows.
2. C:\Users\UserName\AppData\Local\Microsoft\WindowsApps\iTunes.exe” /setPrefInt carrier-testing 1
3. The command does not return anything. Note, it returns nothing even if the itunes command does not exist. Please make it sure you have your itunes on the directory, because it seems the location of the itunes is system-dependent.
4. Download the Carrier setting file
• for iPhone6s iOS12 or 13: Docomo_6s_ios12_13.ipcc
• for iPhone6s iOS11 (from 11.3.1): Docomo_6s_ios11.3.1.ipcc
• for iPhone7 iOS12_13 (worked at 12.4) Payload_78X_ios12_13_Docomo_jp.bundle.ipcc
• or make your own:
5. Run iTunes and Connect your iPhone to iTunes.
6. iTunes:
1. Click the "iPhone" button.
2. Shift+Click "Recover iPhone."
3. change the file from * to *.ipcc.
4. find the file: Docomo_6s.ipcc and click ok.
7. Rakuten band 3 will be found in 1 min or so. Enjoy Rakuten Mobile UNLIMIT!