There is a limit on how many people a 4G carrier can support at a given location. That’s why you might find it impossible to get online or even send out an SMS when you’re in a large crowd such as in a concert or at year end street party. Switch to 3G to get a better connection.
To switch to 3G on iOS, you have to disable 4G. Go to Settings > Mobile Data > Mobile Data Options. Under Enable 4G, tap Off to switch off 4G.