To use your gateway with a sim card you will need to have the NA502S gateway and a sim card without a pin. To disable pin from a sim card you will have to put it in a phone and manually remove the password according to the procedures of your phone model and make.
To start using your non password protected sim card you have to turn the gateway fully off with the switch inside. You can then insert the sim card and turn the gateway on. The gateway will then find the correct settings for your sim card and automatically switch to cellular network when it detects that the ethernet connection is down.