Update...
ATT did whatever they needed to do on their end, but my APN settings were not set correctly. I found some info on the web where someone had to reset their phone to factory specs to get LTE working. I didn't want to do this so looked for additional APN settings and found the following threads:
Requesting AT&T APN Settings for Galaxy S4 - xda-developers
AT&T APNs for custom ROMs - Vivid - RootzWiki
I kind of mashed together a couple of different versions but based mine off of the second link's APN #3. As soon as I entered and saved, the phone instantly recognized LTE and I was getting monster speeds here in NYC.
Here is what worked for me:
1. Call AT&T tech support, tell them your LTE isn't working, and they'll most likely need to update your account to an LTE 4G data plan do some other back end stuff
2. Make sure the store gave you a sim card that works with LTE
3. Call tech support again (and again, and again) and confirm that AT&T has updated everything on their end (you may need stay on the line and work with a tech support specialist who will take care of the IMEI business)
4. Go to APN settings... menu > settings > more > Mobile networks > Access Point Names
There should only one option that's already checked off. Click on whatever the title is (you will be changing it) and it will take you to the Edit access point screen.
These are the settings that work for me:
Name: AT&T LTE
APN: pta
Proxy: Not set
Port: Not set
Username: Not set
Password: Not set
Server: Not set
MMSC:
http://mmsc.mobile.att.net
MMS proxy: proxy.mobile.att.net
MMS port: 80
MMC: 310
MNC: 410
Authentication type: None
APN type: default,agps,admin,fota,mms,supl,hipri,internet
APN protocol: IPv4/IPv6
APN roaming protocol: IPv4/IPv6
APN enable/disable: APN enabled (this should be greyed out)
Bearer: Unspecified
One last note: I downloaded Notification Toggle app from the Play Store. When installed, you can make the "4G" options menu accessible via the notification bar dropdown. When you go in, make sure your preferred network type is set to LTE/GSM auto (PRL).
And...that should be it! Hope that works for someone else out there...