AT&T galaxy s3 3g

[SAdata]

Hi there. My friend has the at&t version of the Samsung galaxy s3 - the i747. Since coming back to SA he can pick up 4g and 2g on Vodacom. But he can't pick up 3g at all?? The specs of the phone say it has 2100mhz 3g and 2100mhz 4g. I know voda has 2100 3g but I thought it's 4g was on 1800mhz. So why does it not pick up 3g? It either says 4g or 2g? Thanks so much