How can I make a fixed image work on mobile?

Here is my read only link:

On Desktop it works perfectly but on mobile the fixed image scrolls with the page like a normal image. I’ve read a few solutions on the forum for the fixed images on mobile but I’m not quite sure how to incorporate them with the layout of my website.
This is the website I got inspired from: Somehow on mobile it works perfectly for them.
Can you please advise me on a solution? Thank you!

Sorry - but I see nothing wrong with any breakpoint.

Hello Jim, thank you for replying!
The parallax scrolling effect is not visible on my homepage on mobile because somehow on mobile view the fixed background images are not supported. I wanted to know if there is any alternative way to create the same effect (since Jeremiah website made it work)