You may need to reconfigure the radio, as it will probably be configured for the area where the phone was intended to be sold, since this is obviously a grey import. N.B. if the phone does not have FCC certification, it may be illegal to use it in the US, or if it is incorrectly configured. Dial *#*#4636#*#* to get into the phone diagnostics, where you can reconfigure the network radio types. I'm not certain exactly which options that model will have, or what you need for AT&T. Try the 3 dots at the top right, then
"Select radio band", and "United States", if that is there. For network type, my first guess would be "LTE/CDMA/UMTS auto (PRL)", if that option is available; you may need to experiment. LTE is not the same thing internationally, so will only work if the phone has the LTE bands used by the network / country.