Like you I popped my sim card in and automatically connected to HSPA, but no LTE. Used AT&T's chat feature on their website and talked to a represenative, and within 10 mins I was getting LTE connectivity. No change in Sim card, and didn't have to go to a store.
Karen(The chat lady) didn't offer to turn it on for me. I asked to have the Sim sent. Just for laughs I took a pic of the conversation:
Karen: Half hour drive to your store would still be faster than 3 to 5 days shipping...
Me: I'm in no hurry, been using the phone for over a week. If its free I'll wait for the mail.
Me: Saves me gas.Its in the opposite direction of where I usually drive and the clerks there are usually high.
Karen: Bear with me while I get this set up for you.
Me: Thanks for your time, appreciate it!
(Time Passes)
Me: Is it your birthday?
Me: If it is...Happy Birthday!
Me: These chat windows still strike me as magical. Makes me feel like I'm in Disney World!
(More time passes)
Me: Hope you're in warm weather where you're at. Here it was cold and was snowing today

Karen: Its about 50 degrees.
Me: That's not bad...hopefully sunny. The sun always makes me cheerful.
(Time Passes)
Me: That and the new Nexus 5!
Karen: Verify you're address please.
Me: *address*
Karen: Thanks.
Me: My kid is chuckling at the conversation. I told her you're paid to put up with knuckleheads.
Karen: LOL!!
(Finally I got her to laugh!)
Me: Not that I'm a Knucklehead...but I guess that's your decision.

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