why cant i text on my phone without wifi

First though is making sure you paid your phone bill. That's happened to me before... took phone off WiFi, tried to text, then realized I forgot to make my payment. Second, make sure Airplane Mode (orange button with plane) is off and Cellular Data (green button) is turned on.

If neither of those are solutions, this Apple Support page that might help trouble shoot the issue: https://support.apple.com/en-us/HT204065