You are on the iPad baseband, you can't use any other firmware except custom firmware. So go to
iClarified - Apple News and Tutorials look through the tutorials -> Jailbreak and look for the method that will will create a custom ipsw with the iPad baseband and restore using Option(Mac)/Shift(Windows) + Restore using the custom ipsw. If you need the iPad ipsw let me know I still have it but I think you'll be able to get it on iClarified. When you sell the phone make sure the buyer knows this or they will be very unhappy when they try to upgrade the phone.
