Thanks for contacting TextNow Support!
The very first thing to check is to make sure your Mobile Data is turned on. On the Sharp phone, you can find that under your main Settings app > Data Usage > Mobile Data.
Our phones use the data connection for calling and texting, either through wifi or through the Sprint 3G/4G network. To access 3G/4G, the Mobile Data slider needs to be in the "On" position.
If that doesn't help, the next step is to contact us at TextNow for further troubleshooting, at the number you see below.