Got a new Wi-Fi box and my iPhone can't join it?

Ever since I got a new Wi-Fi modem box, I can't get my iPhone to join it. I'm entering the right password (it's on the back if the modem) and it says "failed to join Wi-Fi" what do I do?

Check to see if there's a firmware upgrade available for your Wi-Fi device. This can fix a lot of problems that are discovered after the device was released by the manufacturer.