Sorry to here about your problems what a nightmare.
I am in the process of moving from Plusnet to 3. We are in a bad signal area and most days I am unable to get service in my home. When I signed up two months ago they did not tell me they don't support WIFI calling which they should have done knowing the signal problem. I rang them up yesterday for the PAC code and was cut off because of the signal problem while speaking to them. Not impressed with the company at all.
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.