iOs mobile Safari has a problem rendering fixed background images, unfortunately. This is an iOS browser limitation.
This issue is faced by many other platforms and every platform has its own workaround for this issue.
Workaround in GoHighLevel
You can have a different section for mobile phone which contains your Parallex Background image and for the mobile phones, you can select any other option than Parallax.
Here is an article on making separate sections for mobile phones:
There are many threads on different forums that address this issue and might be able to help you.