Couldn't find Mobile Network Type

Hello, Recently I have bought an Unlocked At&T LG G3 The seller told me that the phone has a bad IMEI But it's unlocked to be used interntionally. When I received it, It's working fine, except the data network speed trough EDGE is too slow. As yo can see on the screenshot below I couldn't find the option to change the network type. and I'm stuck at "E" which mean EDGE.

http://s14.postimg.org/8r7vs0x27/Qui...11_41_43_1.png

The phone's model is LG-D850 and it's running android 4.4.2 KitKat. it has no IMEI when I dial *#06# it shows IMEI: 0-05 on Settings/about phone/Status... it says IMEI: 0