I’ve the same problem with my 13 Pro. Can see the mast from my flat and had perfect 5G signal when new a few weeks ago, but in the last week or so (possibly since updating to the last iOS) I’ve not had any 5G. Just been to Stratford via Canary Wharf and not a drip of 5G apart from when I followed Tutts suggestion which brought it back but then it went again. Back home I’ve done it again and 5G is working again for now. Did reset of network settings yesterday. Feels like an iOS bug to me..
