The
router's firewall takes care of the open ports

What I think the op should do is, is create a backup of his current MTNL modem configuration. Then perform a factory reset to both the router and modem.In the modem's interface, set the modem to bridge & disable dhcp, log into the router's interface, set mode to pppoe, enter username password, save configuration and exit.Also, right click on your connection icon in
windows, go to properties, tcp/IP properties, set everything to auto, apply and exit. If the above steps are performed correctly, I see no reason why his net shouldn't work.OP, if you aren't able to understand my post, I'll add screenshots from my old
Belkin for each step, shouldn't be too hard, now.