First off, check if 4G/LTE option is enabled in network mode.Next, check if you are registered on the 4G network in the "service providers" option.If you are within the coverage area, then it should connect automatically; however, if you still can't connect then you need to call 310.
Note: A few days ago, the same thing happened to me eventhough I was standing close to a 4G cell tower. It was only when i called 310 that they said, "Sorry to hear this Sir, It seems that 4G was not activated on our side. I have sent your request to the technical staff." Strangely, thereafter the 4G icon appeared instantly :-?.
Sent from my D5803 using Tapatalk