From my experience w/an AT&T LG OPTIMUS G PRO,you won't be able to get the update(s) pushed w/o AT&T service.
I bought the phone used,directly from AMAZON.
Then,I proceeded to SIM Unlock the phone,no problems there,works flawlessly on T-MOBILE.
However,it was running ANDROID 4.0 Ice Cream Sandwich & the only way I could get the 4.1 Jelly Bean update was to buy a months worth of service w/AT&T.(can't remember if any aftermarket/root-ROMS were available).
If it were an unlocked phone,not tied to a carrier out-of-the-box,perhaps/maybe the mfg (LG) could have pushed the update.
I'd contact SAMSUNG & see if they can help you out,or,if you know someone on AT&T who wouldn't mind letting you use their AT&T SIM Card to update.