Doing future stuff in a mid-evil setting would be tough. In that regard, i agree with [@Dinh AaronMk]. Technology from the future tends to be built on other technology. You can't just have a computer, you need a way to power it. So now you have power stations, but you need to be able to build the power stations. You then need people to run the power stations, and people who can build/acquire everything it needs. You want phones or internet on top of that? Now we need to be able to create cable, run cable, and people to understand all of this. 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.

Dystopia is another genre you might be interested in. This genre deals with crumbling societies that can feel mid-evil, but posses both magic and some advanced technology. Hunger games is a Dystopia world.