Hi,
For those having the Sterlite modem, here's the catch, i had it in my office too and it just refuses to work on ADSL+2 Mode, therefore, login to the router, goto advanced management and ADSL, change the mode to G.DMT save and reboot router, it should do the trick. For, if it doesnot solve the issue, please copy paste your SNR margin, etc here for better advise. I would rather suggest a screen shot but make sure you get rid of the macid and ip details, etc in the screen shot.