I have the same exact problem. I have called them up a couple of times and they can't seem to figure out what is wrong. At&t set up a Case Id for me so a technician will take a look into it, but I don't really think that will solve anything.
My guess is that AT&T hasn't provisioned your SIM card for LTE. They have had some hiccups lately in moving people over and activating new iPhones. Since the old iPhone didn't have LTE, the SIM cards were provisioned for HSPA+ and below only.The weird part is, a friend of mine with same Phone has it. Literally, us standing next to each other he would have LTE and I would have "4g". IT is on in settings.
Thank you, I was having a bit of trouble with something that is now fixed, and I noticed I lost my LTE. I was searching everywhere but this site (which I come to frequently) for the answer.Do you have LTE on in your settings? Try toggling airport mode on then off. If the LTE signal is weak, it will default to 4g. You can literally take two steps and move from LTE to 4G. I think the phone is set up so that if the LTE signal is below a certain strength it reverts to 4g, even though that weak LTE signal would be faster than the strong 4g signal.
Were you on a voice call at the time?The weird part is, a friend of mine with same Phone has it. Literally, us standing next to each other he would have LTE and I would have "4g". IT is on in settings.