Hold on. The link you gave for is a wifi router only and not modem+router. You need a modem+router to replace your MTNL provided one and use wifi as well. Search for D-Link dsl 2750U. It has better range than D-Link dsl 2730 and some other features but price difference is not much. Rest all yours choice - brand/model.
They come with 3 years warranty.
They come with 3 years warranty.