Hello,
this might be part of the theme or maybe there is a theme setting to disable this. I do not think that it has anything to do with the myStickymenu plugin at all. You can see in their demo that it also has a sticky menu as you scroll down:
https://fasterthemes.com/demo/rider-wordpress-theme/
You can create a new thread asking the Theme Authors via their support forums.
There’s also some CSS you might be able to add, though, I wouldn’t recommend doing so as it might break other theme features, use are your own risk. You can paste the following into Appearance > Customizer > Additional CSS:
.scroll-header {
position: static !important;
padding: 0 !important;
}
.royals-container {
display: block !important;
}