Data in UK add-on data pack doesn't work - tried everything

I'm visiting the UK and always use my SIM from Three while here. This time I added a data pack (10GB plus calls/texts for one month), but data doesn't work. No apps work, and Safari says "Unable to make a secure connection to server". I've been back to Three multiple times, by phone and in-store, and they have tried everything, including the usual Network reset, escalating the problem to higher up in IT who swear all is well at their end, and a new SIM card, but data still doesn't work. And no silly mistakes like Mobile data turned off etc. I'm wondering if anyone else has encountered this problem and how they were able to solve it - if at all? I have an iPhone SE 2nd gen with iOS 17.3.1.