g3 d850 unlocked cannot detect 3g, hsdpa or 4g lte.

hello! i have g3 d850 at&t and its unlocked but i have problems while i try to connect phone with 3g or 4g lte data, signal bar ddisappears when i force connect with wcdma or lte. gsm work well. apn is ok. o g2 i have 3g 4g lte ok, with same apn and same parameters but on g3 i can connect only with 2g or E. please help me!