Hi! I will personally recommend you to do the following troubleshooting:
1. Open the settings, scroll down a little bit and go to "General".
2. Once you're under "General", go all the way down until you see the option "Reset".
3. After opening this section, you will be able to see the option "Reset network settings". You're going to select this option and it'll ask you to put your security code (the one you use to unlock your phone). You will then select "Reset network settings". After this step, your phone will reboot. Once it's on again, check if your internet is still doing the same thing.
Prior doing these steps, you can check your data usage to see if you have exceeded your 2GB of data that come if your plan.
If after all these steps you're still having the same issues, I will recommed you to contact customer service (if you have a different phone asides your iPhone it will be even better, since they'll most likely ask you to power off your phone a couple times). You can contact them by dialing 611 in your Cricket phone.
Hope this works!
As mentioned elsewhere, are you getting the latest updates? A few other questions:
1. Are you experiencing this problem everywhere or just in some areas.
If it's only in certain areas it could be a signal reception problem caused by being far from a cell tower and/or terrain/building materials hampering the signal.
2. Did you drop your phone or damage it somehow? If yes, it could be the phone itself.
3. How has your phone been performing in other areas. Can you make calls okay or do you get dropped calls? Do you have poor audio connections?
If so, it could be the same problem as #1 a bad connection due to distance and/or interference with a cell tower. You can check your phone's signal strength using these techniques.