Is there a reason you think our world can not be a fantasy world? It has been used in fantasies very successfully time and time again. It is not so unusual a circumstance that a reader would be thrown by the concept. The future being less advanced than the past has also been done a number of times. I doubt a reader is going to be confused unless the story telling is sub-par.See, in my story. It takes place in the future but electricity doesn't work anymore so people have had to revert back to horse drawn carriages and living in villages in stuff. It feels sort of medieval in a way. I didn't want people to get confused because it takes place in the future but feels like the past. The prologue explains what happened and how the world works now, since it technically isn't in a fantasy world but our world.
I personally don't like prologues. I tend to skip prologues unless they are very short and sometimes even then, depending on my mood. Unless it is an unusual prologue with poetry, songs or some other gadget like a prophecy to get my attention. Those I read 100% of the time. If the prologue actually contains information that I need to understand that first chapter, then I get irritated at the author for making me go back and read it. Unless the author redeems themselves with a superior story after that, they don't get another chance. It is my belief as a reader that the book starts with the first word in the first chapter. Throw history at me and I am likely to throw the book to the side and not finish it, as I am not a fan of history. I may, if the author has given me an enjoyable story, go back and read the prologue once I am finished with the book. That is occasionally entertaining.
I have tons of back-story on my current WIP. I know events that happened thousands of years in the past on the planet the story takes place on as well as elsewhere in the galaxy. I have names for groups of people that will never be seen in the finished book. Plants, animals, geography, atmospherics, politics--anything and everything I come up with for why the people react the way they do and how the world works I write down and put in a file. As an author I need to know how my world works and why. But the reader doesn't need my research notes. They need the best story told about the tiny fraction of the world they are seeing at that moment in time.
You are overthinking this, imo. And by doing so, not writing. It's easy to do, worry over this and worry over that and soon you have gone months without so much as writing down a word. I've done it. It won't get your story finished. I highly recommend you sit down and write. Just write. Tell the story as best as you can. Tell it until it is finished. Then you can go back and edit it and decide if you need that bit in the very beginning to lead into the story and explain what is going on to your reader. My guess is that you will find that you don't. But for now, write.
Good luck.