Using iPhone XR model A1984 unlocked in Japan

As I can see the spec given here and the telcom services offered in Japan, A1984 will work. However

IMPORTANT RESTRICTION FOR iPHONE, iPAD, APPLE TV AND HOMEPOD SERVICE.

Apple may restrict warranty service for iPhone, iPad, Apple TV and HomePod to the country where Apple or its Authorized Distributors originally sold the device.

iPhone XR Model A1984 (Referred by you)

• FDD‑LTE (Bands 1, 2, 3, 4, 5, 7, 8, 12, 13, 14, 17, 18, 19, 20, 25, 26, 29, 30, 32, 66, 71)
• TD‑LTE (Bands 34, 38, 39, 40, 41)
• CDMA EV-DO Rev. A (800, 1900 MHz)
• UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz)
• GSM/EDGE (850, 900, 1800, 1900 MHz)

This specific iPhone XR -- the A2106 -- Sold in Japan

FDD LTE (Bands 1, 2, 3, 4, 5, 7, 8, 11, 12, 13, 14, 17, 18, 19, 20, 21, 25, 26, 28, 29, 30, 66);

TD-LTE (Bands 34, 38, 39, 40, 41, 42);

CDMA EV-DO Rev. A (800, 1,900MHz);

UMTS/HSPA+/DC-HSDPA (850, 900, 1,700/2,100, 1,900, 2,100MHz);

GSM/EDGE (850, 900, 1,800, 1,900MHz).

NTT Docomo uses 4G/LTE:

2100MHz Band 1 (Major Cities)

1800MHz(1.7GHz) Band 3 (Limited Area)

800MHz Band 19 (Rural Areas)

1500MHz Band 21 (Not supported by A1984)

Softbank uses for 4G/LTE

2100MHz Band 1

1800MHz (1.7GHz) Band 3(*)

900MHz Band 8

TD-LTE 2.5GHz Band 41(*)

KIDDI au uses for 4G/LTE

2100MHz Band 1

1500MHz Band 11 (Not supported by A1984)

800MHz Band 18

TD-LTE 2.5GHz Band 41(**)