Initially I got my I537 and heard conflicting reports of DIY SIM unlocking being successful, supposedly some people unable to get the AWS bands. So, I played it safe and got the device unlock code straight from ATT. Everything seemed to work at a cursory glance, got LTE and fast speeds, great! Little did I know that I actually did not have AWS unlocked.
In my room I get nearly no reception and rely on a repeater to get signal and be able to do basic things like SMS, make calls, etc. I noticed that for some reason the I537 was not picking it up.
ATT runs their LTE over some TMO HSPA frequencies (AWS) and the phone by default won't do HSPA on those frequencies. An AWS unlock will allow the phone to do either LTE or HSPA on these frequencies, thus allowing it to talk to my repeater.
I followed the guide at http://galaxys4root.com/galaxy-s4-tu...i337sgh-i337m/ but instead used the UMMS code of *#0808#
It works on the latest stock, unrooted ROM. My device was already SIM unlocked but I don't think that will make a difference for this process. I believe that one would require SIM unlock regardless, though, to use another network.
This guide unfortunately depends on Windows specific tools, so I had to use a Windows machine to do this. It's a pretty easy guide -- basically you tell the phone to expose its modem over USB, use a Qualcomm program to talk to it, do a backup of the NVRAM, and NV Restore someone's I337 NVRAM that has the AWS enabled.
It works perfectly for me and has worked for others as well (see comments on that guide). The only difference is we have to dial *#0808#
You may want to add the T-Mobile LTE APN as well:
Name : T-Mobile LTE APN: fast.tmobile.com MMSC: http://mms.msg.eng.t-mobile.com/mms/wapenc MMS Protocol: WAP 2.0 MCC: 310 MNC: 260 APN Type: default,supl,admin,dun,mms