Hey there! It was so nice of your brother to give you his device! Sorry you're bumping into issues using it!
Was your brother using this device with T-Mobile service? I know it isn't equipment that we retailed, but he may have unlocked it, so I wanted to ask. Although we don't have the PUK unblock steps for a ZTE Maven like this one, they should be very similar to the steps for PUK code unlock: ZTE Zinger. My only concern is that you'd need to Contact Us to ask for the PUK code before you can successfully change the PIN, and we may not be able to provide it for non T-Mobile equipment, or equipment that hasn't been used on our network.
Your question about factory resetting the device is a good one - I'm honestly not sure if that will take you back to a start screen but if you haven't personalized the equipment yet then it's totally worth a shot! Let me know if that works!