@mariam222 Yes I have the data on, and I've tried restarting the phone multiple times. I have more than enough data and the exact APN settings as other Samsung Galaxy S3 Mini owners who's MMS messaging works.
You have clearly exhausted all the usual suggestions , so time to try something different.
I have used android phones for many years and always had problems with the stock android messages app.
The solution is to change to another from the Play Store.
From experience the following will send MMS: