iPhone 13 unexplained data drain.

I’ve had my iPhone 13 for a while now and always left my data on as with my previous iPhone with no problems, as do other family members with iPhones. In March on 2023, I suddenly saw my data being drained without explanation. Until this point in time I used very little of my 6GB data. My data use habits did not change in any way. All my research could not establish a reason that this was occurring. I now have to turn my data off along with all app use privileges at all times this drain, only turning it on when I’m away from wifi and only for the period that I need to use it. If I leave the data on, even when I’m on wifi, this drain still occurs. The other night I forgot to turn the data off and 1.5 GB. Of days was used, even though no one was using the phone. I have turned off all data use options such as use data when wifi is weak, etc., etc. I’ve also turned off iCloud as I read where this might be a cause. This doesn’t seem to stop the drain. Has anyone with similar problems found a cause and fix?