Fixed Background effect not working on Ios (Iphone) or small browser window

Hello

The fixed background effect in the link below does not appear on Iphone, maybe on other IOS or smartphone devices too. Same goes when the browser window size is decreased to a certain point having a PC with Windows 10:

https://rud-arch.levindelamour.com/

So the question is how can this be fixed, for exemple by adding custom CSS code or even using JavaScript?

Regards

Julien