No cellular signal. No carrier tab in settings? Sim OK

Hi folks, during my holiday in Spain my phone struggled to find any signal on the 4th day onward.

I waited to get home to see if that solved it and still no better.

24 hours later, magically my network appeared, but only for 2 hours before dissapearning again.

The Carrier Tab is also missing from settings.

I have done several resets of network settings, DFU reset and and reset to factory settings.

The Sim works fine in an older iPhone I have.

Any ideas?

I have fixed several iPhones before in the past but I couldn't really find an iFixIt article specific to replacing the Cellular antenna?

Any help very much appreciated.

James