AT&T uses LTE band 1 for service.
since E210S only supports band 3 and 5, and 3 and 8 for E210K, there's no way to access AT&T LTE network with your device.
It is reported to support band 1 for newer devices like S5 and G3 and AT&T LTE devices might work in Korea since May 2014,
LTE won't be available with Korean S3 in US.
Moto Glam (XT800W, sold & bought again - CM10)
Motorola Defy (MB525, broken)
Google Nexus S (broken)
Samsung Galaxy S2 (SHW-M250S, radio broken)
HTC Chacha (A810e, sold)
Sony Xperia mini pro (SK17i, broken)
Google Galaxy Nexus (Currently using - Slimkat 4.4 ROM)