DLink has regional products that are designed to work in specific regions or countries. I presume that if you take one region product to anohter, theoretically it should work however may not due to differences in WiFi and wired standards. I don't see the DIR-857 listed for Argentina:
http://www.dlinkla.com/detalle/familias/61However there maybe other problems going on.
What ISP modem Mfr and model # do you have?
What ISP service do you have? Cable or DSL?
Use InSSIDer application to see how many other WiFi routers are near by. How many are there?
What wireless modes are you using?
Try Mixed G and N or Single Mode G or N.
Try changing to a open or unused channel on either.