Did you just purchase the LG Optimus G Pro or have you tried to switch carriers?
If this LG was originally a Verizon phone and you're trying to take it to [virtually] any other carrier than the SIM won't be recognized as the phone will be incompatible with the carrier and towers that the carrier uses. Verizon uses CDMA (Code Division Multiple Access) while every other company uses GSM (Global System for Mobiles). Due to that, trying to transfer a Verizon phone (except for a select few phones) is impossible. If this is the case, try and return the phone or get your money back if you can.
If this is a newer phone or bought off eBay and it's not Verizon, is the phone unlocked? Generally, you'll need to have the phone unlocked in order to get your new SIM card to work with the intended company.
Those are the best tips I have for you (based off my own experiences). If neither of the above helps/works, we can work out some other solution to the problem.