I have an iPad 4 model #1459(wifi/cellular) tied into AT&T/T-Mobile my carrier is Verizon. Is there anyway to change my ipad to Verizon? I dropped in a Verizon sim card but got an error message "Sim is not Valid". It states insert SIM card from a supported carrier or request that this iPad be unlocked by your carrier and to contact Apple for more info.
Why not work through this checklist from Apple before contacting either the Carrier support or Apple Support?
Since you have a specific error message, you might also jump to one of the last steps first. Back up the iPad and restore it to factory new settings and re-test the SIM card that you think should be active.
I've needed to call Verizon to get a SIM card to be part of a shared pool of data, but any active Verizon SIM should work, so once you rule out the iOS software - re-verify that the SIM you have is truly active.