you can use the old bsnl
adsl router but u have to set the connection type as
mac encapsulation in the router. And then do not try to login using portal page, set up a dial up connection in your pc and use that connection to connect with internet(watch the tutorial in youtube "how to set a dialup connection for beam broadband").
Right now i am using beam broadband 10mbps with my bsnl
Nokia siemens networks adsl router and i am able to connect with all of my devices like two pcs and smart phones.
connect your internet cable to on of the lan ports in the router, and set up your wifi settings.
after that conncet to internet using dialup, thus ur pc ready to connect the internet no neet authentication....then using your router wifi connect with your mobile and after that open portal page in the smart phone(it requierd portal login page authentication).
then your ready with all your devices conncet with internet....no need to buy new router...if any quary mail mee....
note: i have tried this with bsnl nokia siemens router with
windows 10 os and beam broadband conncetion...