The iPhone mobile data usage skyrocketed to use 1GB per day whereas I am normally a 5GB a month data user. the culprit was Apple System Services General there is currently no iPhone functionality to turn off System Services Genera. the solution was to block updates.cdn-apple.com using data sniffer called Proxyman.
Tip hat to the person who posted “High mobile data usage on iOS 18 and how to solve it” on Reddit.com
This is something apple developers maybe should look at