First you need to make sure the hardware is compatible with the network you're trying to use it on. If that is compatible and the 4G bands used by Straight Talk are compatible, then you need to check with your carrier to make sure your SIM and device ARE activated for mobile data, that your mobile network settings in your phone are not set to 3G only, and yes, that the correct APN information (comes from your carrier) is also set up (if you only have Verizon's info, for instance, a Straight Talk SIM wouldn't work with those).