Please make sure you sms "BESCHERMING AAN" to 1266 in order to avoid that in the future you will ever be charged for such dataroaming costs again.

Could it be that your phone automatically switches to 4/5G when you are outside of reach of the wifi network or that the strength of the wifi network is to low resulting in the phone switching to 4/5G?

Welcome to the community @ashmilliona, I understand that is a surprise you didn't need. If you were using Wifi all the time, it is possible the phone switched to mobile data when the Wifi network was not sufficient. It is called Wifi Assist on iPhone, and this Apple help page explains how you can turn it off: About Wi-Fi Assist.