The problem is your fixed position object is below the screen's viewport height (the blue dotted line), so it will stay there, and will never come up in view to become visible. If you want a fixed object to be visible, it has to be above it.
What you actually want is the tabs to be scrollable, and to become fixed after they are scrolled into view by the user. Unfortunately, Xd doesn't offer a solution for that natively. I've been using a workaround for this to replicate it, which is a bit hacky, but it works for cases like yours. Here's a video I made on how to do it.
It might be because your prototype is taller than the available space on the phone you are viewing it on, so it cuts the bottom part of the prototype. You should view it on a smaller zoom percentage, not full screen, or just make the viewport size shorter, so it fits on the smaller phones.
Keep in mind on the phone browser, you have an address bar and other stuff, which reduces the available space even more.