To my knowledge, the AT&T S4 doesn't have the hardware to support working on T-Mobiles 1700/2100 AWS HSPA.

This is taken from Samsung's official site about the device:

It does look like it would support T-Mobile's refarmed HSPA on the 1900 MHz band, as well as their LTE though.

 

I know this is an old thread. I'm posting it for the benefits of others.
I live in the U.S. but my phone is the Canadian version of S4 (SGH-I337M).
To my knowledge, the only difference between this one and the AT&T version (SGH-I337) is the Canadian one has unlocked BOOTLOADER (don't confuse it with SIM unlock).

Since I do have unlocked bootloader, I have installed custom recovery, TWRP, on my phone. In order to use QPST to add AWS to my phone, I had to temporarily go back to the stock recovery to "enable cp logging". Without this extra step, QPST simply cannot find my phone.

Further, many of these tutorials are still teaching *#7284# to enter the configuration screen. On my phone it is actually *#0808#.

I am not sure if the AT&T S4 really doesn't have the hardware to support AWS, or AT&T has programmed a baseband without AWS support.

Since I was already getting LTE at 60mbps before (Canadian S4 on T-Mobile), though I did finish this exercise of adding AWS, I have not seen any performance improvement. It was a good session of tinkering though.

If you are on custom recovery and have to go back to the stock recovery, just a reminder when your attention is so warped up with this AWS challenge, don't forget Odin cannot flash a recovery.img file (which you extract from the stock ROM). I should have known better. After some frustrating moment, I archived this file into the .tar format and Odin flashed it without a glitch.