I have managed to use my ZTE ZXDSL 531B modem and router bought from BSNL as only router in a limited way.
I have just connected LAN cable from my local ISP to modem and disabled DHCP in modem.
Limitation: Can use only as on demand dial. I had to make a new broadband connection that requires username and password from network connections and connect with my user/pass. Downside that only one device can be used at a time. Solution to this is virtual wifi router in a windows PC but that has to be on for all time to connect with other devices.
If anyone has ideas, pls let me know.