cannot connect to mobile network but sim works in anither iphone

My iPhone 6 and sim were working perfectly until 3 days ago when I suddenly couldn't connect to a mobile (cellular) network. I had not dropped or misused the phone. The sim works perfectly in my daughter's iPhone 6. I have tried all the suggestions on the apple help page including switching on and off; switching airplane mode on and off; checking for phone and carrier updates (both up to date) and even backing up and restoring the phone from factory settings. Any suggestions?