I have a verizon galaxy s5 sm-g900v that I have an AT&T sim in & it works fine. all you need to do is put in an AT&T APN IN.here is what I have in mine.
name - att
apn - pta
proxy - notset
port - not set
username - not set
password - not set
server - not set
mmsc - http://mmsc.mobile.att.net...