You may think of instantaneous as being 90 degrees away on a spacetime diagram. But someone else's frame of reference will observe your instantaneous at a different angle than 90 degrees on their own spacetime diagram, perhaps they'll even see it as going backwards in time. Their frame of reference is valid too. So if they sent a message back at a 90 degree angle on their own spacetime diagram using the same mechanism they'd be sending a message to us back in time in our frame of reference. It gets really funky when you do this with 3 different observers all observing each other communicating backwards in time to the point where they can cycle a message between the three of them that goes further and further back in time.
You'd have to first throw away frames of reference to make such travel possible but that's the entire basis of relativity and good luck finding a better explanation as to why light always appears to go at the same speed for any observer in the universe.