Mobile Network settings:
Settings> Wireless & Networks> Mobile Networks> Access Point Names> (Network APN) and you will be presented with the following fields:
You will need to get the relevant settings from whichever network you are on.
Enter them manually exactly as your network provides, hit menu, save and restart your phone.
If you are moving network and your phone has settings from a previous network, it is better to delete the previous network’s APN and create a new one altogether.