Default SHV-E210L Having signal/data issues

I have the LGUPLUS korean galaxy s3 and am using an att simcard with it. Im not entirely experienced with phone tweaks so i was hoping someone on here can help. When i boot up my phone it will try to find signal, when it does the signal is usually low around one bar and is stuck on G data instead of 4g. Is there a fix to this? Or is there any modifications that can be made to allow the 4g data? Ive messed with he hidden menu but changing the settings didnt seem to help the extremely slow data issue. Any help is much appreciated.