How to stop iphone from using mobile data for system services

How do I stop my iphone from using mobile data for system services while on WIFI?

All my push notifications are using mobile data instead of my WIFI. How can I prevent this? Including DNS services.

Wifi assist is off, I only have safari and messenger using mobile data as that's how bad the data drainage is becoming.

I have to keep topping up as it drains all my data, even if I'm on wifi the whole day.

thank you!