Short answer is you need to factory reset your device whenever EE decides to change these backend settings.
Basically EE changed some APN settings. Providers tend to do this without thinking. Last time it happened I was with plusnet (which runs on exact same EE network) and, fortunately, they could revert the changes that were pushed to my device. Problem is I don't have access to another phone this time.
Unfortunate, but it seems these companies can't afford proper deployment teams or enough support staff to fix the issue. With Plusnet I had to call back 5 different times, each time they said the same "restart your device and call back" speil, and became irritated when they realise I've already tried this and they can't just drop the call.
EE support team, as you can see in the above chain, is incredibly unhelpful and unknowledgeable in these situations. Their top answer is redirecting you to another team which, if you can't make outbound calls with your SIM, is a totally worthless contribution.
