Probably the software update has relocked your phone to it's previous network. You must unlock it again. If you carry out any temporary unlocking method, your iPhone will get locked to its previous network on every update. To avoid this hassle you need to unlock your iPhone using any permanent unlocking method. For example, you can visit TheUnlockArena.Com and get the remote unlocking service for your iPhone. This method of unlocking is safe, simple, affordable, quick and permanent. It doesn't require a jailbreak too. Good luck.