[quote=@BrokenPromise]
A far simpler solution would be just to make a device that resembles a computer that neatly fits into the fantasy setting. Maybe there's a tablet with runes etched into it that rests in front of a lake, and the lake's reflection changes as "commands" are entered in when runes are pressed.
[/quote]

At this point we're probably not going the route of medieval-modern like OP is proposing but general pop-fantasy. Something like this too may not be able to proliferate among the common people like it does in the modern world only lending to that more and not changing the paradigm of society so that it's like a modern society.

It would be like The Witcher, it handles the medieval society really well and manages to have blended in anachronisms associated with the modern world. But yet it's still very much a medieval fantasy world.