I have had this problem a couple of times after changes to my plan. After having network settings resent several times, as advised, the internet was still not working. with error messages popping up. I was given this different solution a couple of years ago and it has helped again this time so I thought I would share it as it seems to be a recurring problem. On your phone, go to Settings, General Management,Reset, Reset Mobile Network Settings and this should sort it out. Mine is an android so I don't know if it is the same for iphones.
