Could you try these steps,
Wait a few minutes after each step and Try some texts,
To test, I would try with some numbers that aren’t included in your contacts
Text the word NUMBER to 2020 and also to 43430
Send a text to your own number
And to these numbers
If those work, try and send some texts to a few contacts
(Reset network settings)
Be aware this will erase your saved wifi passwords, it won’t erase or affect anything else)
Go to Settings
Click General management.
Click Reset network settings.
Click RESET SETTINGS.
Click RESET SETTINGS to confirm
(change network mode)
And select 3G /2G auto or 3G only
Go to settings
Go to network operators
Make a manual search and select a network other than o2
Wait till it fails then select o2
Install this app
Open in and click to use as your default messaging app
Try a few texts, if they send,
Go back to your default messaging client and send texts to the same numbers