How to speed up bsnl 2g internet in dlink dwm 156?

I am sure it is now toast and you cannot downgrade if the wrong firmware has been installed.
I would ask D-Link.

It's most likely because 'Telephony' is turned off in your PC. Just search Google for Error 711, you'll find Microsoft's solution foir error 711. Just do as directed. USM modem will dial in no time!

http://support.microsoft.com/kb/122839

check, there is solution

http://www.smc.com/index.cfm?event=downloads.searchResultsDetail&localeCode=DE_DEU&partNumber=3235&docId=1378&modelNumber=1506&KNOWSPARTNUMBER=false

So, If you have internet connection speed problem with your wireless try with the following steps,
1.) Reset the wireless router by using the power switch on it or by unplugging it and plugging it back in.
2.)Place the wireless router in an ideal location. The higher it is, the better, but don't put it near metal objects.
3.)Get as close as possible to the router with whatever device you're trying to connect.
4.)Keep the router and your connecting device away from other electronics. You should especially keep away from phones, fax machines and microwaves. etc.,
5.)Adjust the Wifi antenna if it has one.
6.)Change the router's channel, especially if other wireless routers are nearby.
Following these steps would give you stable internet connection and also increased speed in connection. You can check your internet speed from www.ScanMySpeed.com before and after following these steps.

Error 633 D-Link (DWM-156) modem

-my d link usb modem dwm -156 3.75g cannot detect the computer

I would suggest that you go through this guide, it will explain you every step which you need to make to get your modem and internet connection back on track:
The ultimate Link Troubleshoot guide: Problems. solutions, PDF manuals

restore your system or repair/change your os... if still doesn't work den change your dwm 156

you will need to go to the D-link site and download the updated drivers if available.

Here is the link for D-Link
http://www.dlink.com/default.aspx
enter you model number in the search field and see if you have updated drivers available.