My carrier is Straight Talk (using ATT network) and they require the user to enter the APN settings in order to access their data network. My Iphone 4s is factory unlocked but I can not get to the Cellular Data Network tab to enter the settings?

This iPhone was purchased where as factory unlocked?

If factory unlocked, the option to enter APN settings would be available.

What is preventing you from selecting Network on your iPhone at Settings > General > Network?

Ithink this is the problem with all the un-loced iphones.its a big rip off.you buy the phone,they hook you for couple years,then you go through the thing called"att unlock" and then the darn thing still won't work.then after a couple hundered hours on the internet i find out,att still runs it.tthere all in on this big game,att owns streight talk!! and the only way to get everything to work again is put it back on att again! or jail break it is what streight talk told me insteed of helping me with the problem,

Yes ATT hides the APN settings so when you try to use a st sim it automatically reads on AT&T network so the APN settings are hidden and cannot be accessed however you can go to apn.straighttalk.com on you'r phone and select APN/data settings then select AT&T then iPhone and a screen will pop up asking if you would like to install the profile for whatever reason it's not verified but works fine. You can also go to www.unlockit.co.nz and set up a straight talk APN profile that is verified not sure exactly what that means but both work to get the data working on the phone for straight talk but it does not make the MMS work. There are a couple other methods I have read about one is on howardforums using ibackupbot to manually change the files on the phone I haven't been able to get mine to stick using that method but other people have and there is a sim swap method to trick the phone to reveal APN settings using a tmobile sim I have not tried this but other people have with success. The last two methods mentioned will make both data and MMS work