Thanks for letting us know about the trouble you're having. You should have the option to turn that feature off within the Settings of the application:
TextNow Menu > Settings > Calling > WiFi to Data Handover > Off. This will help the issue in the meantime.
I'd also advise making sure you're running on the newest version of the application (you can check for updates within the Google Play Store.)
In addition, I have sent off an email to the address we have on file. I'd just like to follow-up with you on this one and make sure we get the issues rectified.