If you're using cellular data, switch on 3G. You should see the settings at settings>cellular+SIM, or switch to a SIM from another network for cellular data.
You should see whether you're on 3G or not if the icon beside the signal bars is H/H+. G/E denotes 2G data.
If you're using Wi-Fi, perhaps something with your router is causing it?
You could try a reboot or a soft reset (volume down + power button held down until screen turns off), of course.