According to 18:96-98 and 21:96 we must find a big wall made of iron and copper between two mountains, and we should find it standing even today, as it will remain until the End Times. Where is it located exactly in our planet so I can go and see it?

Nowhere in the verses you mentioned was there an instruction to find the wall, nor were there instructions as to its location. Common sense would tell you that the wall would most likely be covered, seeing as how thousands of years have passed. There are huge expanses of the surface of this earth that have yet to be explored, some of them mountain ranges. Perhaps in your case sense is not as common as one might think.