About Wi-Fi Assist - Apple Support
You may need to tell your phone not to use cellular data when it is having problems connecting to Wi-fi. I am not sure but you may be able to go into Settings somewhere (I have no cellular service on my phone so I didn't pay much attention to this) and individually customize which ones can use cellular data.
Also consider your usage of these services when not connected to Wi-fi:
What can be labeled as Apple Services transactions
Music, movies, TV shows, stickers, books, and app purchases (from the iTunes Store, App Store, and Book Store).