I had to get a new sim to replace a lost one. I am also using an old iPhone 4s handset until I can get a new one. The handset is on the most recent iOS available to it which is 9.3.5.
The SIM let's me make calls and text but no data even though it says 3g on the status bar.
I deleted an old talk talk profile from the general settings. There is no option under mobile data to manually enter the apn settings.
I have tried all the turn on, turn off, remove SIM, reset, reset network settings, manually choose carrier all to no avail.
Customer service just say it's something that the engineers are working on and that it's not a common problem. Evidence on the forum would suggest otherwise.