hey folks just posting because I successfully injected my girlfriend's IMEI without a backup and got 4g back as if nothing was the matter. AT&T SGS3 here.
The whole process took me about four hours of trial and error, but it all worked out in the end, and peoplearmy's method works fine if you take each step carefully.
Here's what happened to me with her phone:
Flashed insecure boot for root, CWM, and busybox (all-in-one) by SGS3 Toolkit, did a nandroid.
Flashed MIUI, didn't like it.
Flashed AOKP jellybean.
Lost her IMEI during one of those steps. I realized after installing AOKP that she wasn't getting data, or at least it appeared to be stuck on "E" 95% of the time. Whenever she would reboot her phone, she would get 4g for a couple minutes before it would vanish. Her IMEI in About Phone > Status read as "0" at this time. ..restoring it to the first nandroid didn't restore her IMEI.
I tried unsuccessfully for awhile trying to get her phone into DIAG mode or whatever with the AOKP rom, but I couldn't get the scripts to execute properly in terminal emulator or perhaps I was doing it entirely wrong. Whatever the case, I restored the phone back to my first nandroid of the stock-rooted rom so I could use the stock dialer. Seemed like dialing *#7284# in the stock dialer was much easier, LOL.
Anyway, I put the phone into RMNET + DM + MODEM mode just fine. I made a backup of my QCN with QPSD and then opened the RF_NV_Manager. I searched for item number 550, but the key did NOT exist on her phone. Following instructions, I added the item. The nine boxes had no values.
Okay, so the thread peoplearmy started has "The IMEI is also listed in a special order so you must follow this guide to ensure your IMEI is correct." but there IS NO GUIDE or LINK as of yesterday other than the quoted text so this is the part that took me FOREVER to figure out. Lol. I tried writing random numbers and ****, and they would get her 4g LTE access for about 15 minutes before it would stop working again... eventually I figured out you need to reverse the numbers every two digits starting from the end, and add "a" to the last one.
not sure if posting her IMEI is a good idea so I'll do an example (your IMEI number is written underneath your battery). It's easier to do bottom to top in my opinion.
which is easier read as
6 57 09 10 51 09 42 76
should be input into the nine boxes as:
(first box is always 8)
1 (the 01 will make the zero vanish so don't worry. this threw me off at first.)
I don't have any clue as to what the original post or guide said about putting your numbers in properly, but this worked for me. It wrote successfully with RF_NV_Manager to her phone and I rebooted it. AND 4G WORKED!!! And her IMEI number in Settings > About phone > Status matched exactly to what was written under the battery!!
And uh, I didn't use the SGS3 Data Restorer App at all. I don't really understand what its purpose is, but perhaps others have used it to great success.