Default AT&T S3 I747 LTE Problems

Today I just changed my Motorola Razr HD XT925 for a Galaxy an AT&T S3 I747 because I wanted a phone that would have working LTE on AT&T but there seems to be a problem with the unit. I can only get 4G but no LTE. Device settings code *#*#4636#*#* and it is on the GSM auto(PRL) I tried to change it to LTE/GSM/CDMA or LTE/GSM but it changes back to GSM only.
The phone is on latest Stock 4.3 firmware
What am I doing wrong?