I just received my Rogers SGH-T899M and can confirm no LTE on AT&T. I've tried playing with the APN settings to no avail. Rogers site listed it as support 5 LTE bands while Telus and Bell list it as only AWS LTE so I made sure to get the Rogers version. I guess the Rogers site was wrong. No LTE on AT&T in Minneapolis.
I also can't get internet sharing to work. Just says there is no cellular data connection even though the browser works fine.
The manual and box both say single-band LTE, so ALL SGH-T899M only support AWS LTE. This is definitive.
It will work on AT&T LTE in places with band 4 deployed, unfortunately that is a very small subset of AT&T's LTE network and it is quite difficult to tell which area runs on which band.
Disappointed. Back to my Galaxy Note 2 I guess. Frak AT&T for disabling tethering unless you pay ridiculous amounts of money. No way for root my 920 to ignore their stupid instructions.
EDIT: I was able to get internet sharing to work! Here's how!
Go to the market and search samsung electronics. Download the "apn" app.
Now go to settings and open the apn settings. Click edit apn, then click ATT Broadband. Change the APN from broadband to pta (if you have an LTE plan) or to phone and save. Internet sharing should now work. Great thing is, it doesn't ask AT&T for their damn permission!