How to enable AWS band on Galaxy s4 SGH-I337M?
In addition to changing from *#7284# to *#0808#, I also had to temporarily go back to the stock recovery from TWRP so that I can "enable cp logging".  If I don't do that, QPST (the program you have to run to add AWS) simply cannot find the connection to my phone.   

Originally I was afraid that going back to the stock recovery would remove my root, and I would have to re-root again before I can install TWRP.  It turns out a non-issue.  

Just a reminder when your attention is so warped up with this AWS challenge,  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.