Losing 3G settings After restore iPad 2 3G

My iPad 2 3G used with an unlimited AT&T data plan and works fine. After I did a restore to iOS 3.22, the iPad lost the settings for cellular 3G service completely and couldn't recognize the SIM card. It just turns into a wi-fi only iPad.To fix it, tap Settings > General > Reset Network Settings. And use the SIM eject tool to open the SIM tray in the iPad 2, remove the SIM card and insert it