If you have movistar plan in argentina and you see a ! at the right of signal icon, it means your apn is not set right.
To solve it:
Go to AJUSTES - MAS - REDES MOVILES - APN
Open every configuration of apn, look for proxy ip address and just delete it, do the same with port. Go to 3 dot icon and save.
Repeat this process with every configuration.
Restart your phone and volia!USEFUL

Thanks for you tutorial! It's now highlighted

Great!