Hi there
I’d be happy to help with your LTE access! Was this iPhone 5s purchased from AT&T or is it unlocked from another carrier? Also, if you are using the SIM card that was in your old non-LTE device, it’s probable that the SIM card is the cause. We would just need to replace the SIM card, which is free from any AT&T store!
I hope this helps!
Charise