The culprit in my case was iCloud Private Relay.
I had the same problem, to the point of going crazy. I tried many of the solutions described here and on Reddit. I disabled Apple Intelligence, removed dictionaries, disabled and then re-enabled iCloud. I reset the network settings, and even the whole settings, as Apple had suggested during a support call, but that never really solved the issue. Sometimes I was using up to 20GB a day.
In the end, I had to completely reset my iPhone, with the pain of reinstalling dozens of apps, wallet cards, and all settings. Actually you can avoid this. Ok, for two weeks everything went fine, but afterwards that’s when I finally found the culprit.
I turned on iCloud Private Relay after roughly two weeks following the complete reset. A few minutes after activating it, I noticed that the problem started again. And I realise that the problem actually started on my previous configuration shortly after having enabled it.
My recommendation is that it is better to use a standard VPN if you really care about keeping your browsing private. iCloud Private Relay is sometimes poorly configured, incompatible with certain networks, and it obviously indeed caused this excessive data consumption.
