Why no 4G on by BYOP AT&T iPhone 5's

I just signed up with Straighttalk BYOP with my two AT&T iPhone 5's. I had 4G service before, as there is no LTE in my immediate area, but now only showing 3G with Straighttalk. Why?
https://straighttalkwirelessforum.com/
https://straighttalkwirelessforum.com/viewtopic.php?f=6798&t=1267432