This month my data usage is very close to my plan limit, so I am trying to limit usage to prevent going over. But every day I notice it is still creeping up, and when I look at the log my provider shows me of data used, I see lots of (mostly) little transfers at times when I know I was definitely in range of WiFi. I am especially sure of the ones in middle of the night; these are something like 20-50kbytes of data, so I was wondering if I am missing a setting that lets the phone switch over the the (lower powered?) LTE after being asleep for a certain amount of time which I could switch off, or if there is some other reason that it isn't using the WiFi connection.
When the phone is asleep (also known as hibernation mode), the WiFi radio is shut off after a time to conserve battery power. Anything that occurs on the phone, such as receiving notifications, background app updates, et cetera, uses the cellular network instead of WiFi. The one notable exception would be iCloud backup, for which the phone wakes the WiFi radio.