iPhone 14 pro WiFi connectivity problem iOS 18.3.1

I have just had problems restoring and updating an iPhone 14 Pro, eventually sorted by Apple staff at their store. On getting home it shows that it is connected to our home WiFi, but had an E where the WiFi signal icon should be, and no internet connection.

I did an internet reset. Again the WiFi symbol appeared and then immediately disappeared leaving the 4G symbol.

I turned off Mobile Service, and the internet immediately connected. I can't have both on like I can on the 16 Pro or could on iPhone 8 and 10.

How I can get both running at the same time as the user of this phone would have difficulty getting roaming on when they need to for using WhatsApp or maps when out.