Ok. Try this:
Open Textra. Go into settings and look for a way to restore defaults.
Then go back into Textra settings, then MMS, then Advanced. Once there, make sure the setting says "Carrier has no limit".
Also, make sure your Data is on in the phone's settings.
You might want to go into Settings, Apps, Textra, then storage. Once there, clear the cache and force stop the app. Swipe everything closed.
Try sending a picture.
If that doesn't work, you might want to download another messaging app, may Android Messages, and try sending a picture with that. If neither work, it's most likely an APN issue.