You might be onto something.

I definitely got the feeling that the mastermind behind it all is not part of the main group of hijackers, because of that one guy’s attempt to reach someone on the phone.

Kind of reminds me of how Abbas was the mastermind behind the hijacking of the Italian cruise ship Achille Lauro back in 1985 while not being part of the main group of hijackers.

But there might indeed be a twist.
Initially I thought the hijackers were left hung out to dry because they couldn’t reach the mastermind when they attempted to call them and instead got their voicemail.

But your suggestion that the mastermind might be on the plane, could be an explanation on why the hijackers couldn’t reach the mastermind.
Their phone would’ve been taken away and presumably be off/not connected to the in-flight WiFi that had been off the whole time.

As for who that mastermind could be, I think we’re thinking of the same guy: the curly haired guy that almost missed his flight and that Idris Elba’s character suggested should be let on board despite reaching the gate so late.

Not just because of the subtle focus by the camera, but also because of an unresolved thing that now feels so long ago that I’m starting to wonder if I’m misremembering (I should rewatch the first few episodes I guess).

Namely, IIRC, the guy put his bag/pouch in a seemingly random overhead bin and it is this very bin and pouch the hijackers are grabbing stuff from, like the life bullets we saw the “main” hijacker grab towards the end of this episode.

The fact that the hijackers had to move up the timeline by a few hours because of the blank being found by the girl in the restroom could explain some of the genuine expressions of surprise by the mastermind and why he might not have been reachable, because all of it is happening outside of the scope of his original plan.

The only minor nitpick I’d have is that a lot of airlines nowadays prohibit voice calls via WiFi calling and other apps, and in part enforce that by blocking that kind of traffic on the plane’s WiFi network.

That said, this is a fictional airline presumably with Dubai being it’s home country, so it could very well be that there’s no such restriction in Dubai air travel regulation and/or that this particular airline doesn’t block that kind of traffic on a network level.