If you are on an AT&T Prepaid service. Your plan will NOT work in Japan. It is better just to go with the guides here and get a mobile Wifi or Sim card.
I just had to dig for about 2 hours to find that information because their website had conflicting information that was not straightforward. I’d like to save others from the same trouble.
There were also no recent posts here describing this. The only recent post was on the AT&T forum where the question was asked in July.
Here is the link with the best and most recent information I found as of Oct-31st:
[https://forums.att.com/conversations/plans-features/international-add-on/62d08f33b22ce07ce2acde60#:\~:text=Prepaid%20does%20not%20roam%20in,select%20European%20and%20Caribbean%20countries](https://forums.att.com/conversations/plans-features/international-add-on/62d08f33b22ce07ce2acde60#:~:text=Prepaid%20does%20not%20roam%20in,select%20European%20and%20Caribbean%20countries)