Well, i achieved this. In case somebody wants to know - here is the brief
1. Allow the bsnl router to have wireless-> wireless bridge set as access point and disabled for AP Mode and Bridge restrict respectively. This is your primary router for internet access.
2. Install DD-WRT firmware in E4200. Follow instructions clearly as per the DD-WRT site, start with mini builds before big builds. Duly perform the 30-30-30 reset procedure to clear the NVRAM.
3. Once installed, you will have 2 seperate radios for 2.4ghz and 5ghz. Check the wireless setting and change one of them to CLIENT BRIDGE. Whichever radio you choose, make sure that all its wireless setting, SSID and encryption is set exactly as the one set for bsnl (recommended is WPA2 personal + AEP, the passcode should also match). This secondary router will basically join the primary wireless in typical WDS pattern.
4. Finally chnage the ip address setting. E4200 ip address should be outside the DHCP setting of bsnl router.
To test, connect to other radio of E4200, which was not used for WDS and test the internet. Be patient, it takes about 45 seconds to establish connection. You can see the status -> wirless for the bsnl mac address to see the signal strength. This will confirm that the bsnl router has been detected. All the wired clients will be automatically have the internet shared with them.
Beaware, that this method on E4200 will provide half the internet speed as you normally get is bsnl.Hence this is for casual browsing only, no serious downloading. I use it to connect to my Apple TV 2 ( with XBMC loaded) to download posters and fan arts for my movies stored in portable HDD attached to E4200.
Write on this thread for more details, if required.
