After a week of going back and forward to the Apple shop and spending time on the phone talking to Plusnet, I went online yesterday to see what others said.
The problem: SIM card working for calls and texts but no data. Yesterday I found that I was not alone and others had problems accessing data from Plusnet. I must have wasted at least 10 hours trying to get it fixed.
I just can’t recommend that anyone takes a Sim from them even though customer service was good.
They use the EE network but it seems that something is wrong with their configurations and cannot be fixed on I-phones. There is a fix for Androids but looks rather complicated.