[Q] Who can success Software update(OTA) from oversea(not in US)? topic

Last time When I went to USA for business trip, I Bought ATT G2 (D800) model.
Now i'm living on South korea.

Currently my ATT G2 have v20c version and I want to upgrade for v20y without rooting.

Because all of korean Banking and credit card app run threaten scan and if rooting related files found on phone, it won't run by security problem.

But, When I tried software update every time, it just loop "Not yet registered" popup occur and I won't 20y update from ATT FOTA server.

And I also doesn't have ATT or ATT Go phone sim. If I get or borrow ATT SIM and connect on Wifi, Can I get a 20y OTA from oeversea?

Exactly, Am i need a ATT SIM or Go Phone SIM for successfully get a OTA file ? also that ATT sim must be activated on ATT account ?
I think some of my colleagues have a deactivated ATT go phone sim. But i'm worry that this deactivated sim also can get a OTA from ATT FOTA server.

I need a brothers opinion or success story who can success got a OTA from oversea and not ATT(like T-mobile or H2o or Straghit talk... ) environment.