Unable to connect through VPN

Hi,
I am using D Link's 2640U ADSL2+ router. I am unable to connect to my office network through this modem. I am able to surf the internet with multiple laptops without any issue.
I have also tried to get help from BSNL customer service but to no avail.
Can anyone please help me on this?

Thanks,

Use DMZ option and put the IP address of your computer which one you use for connecting to VPN or use the PortForward and open the VPN port.

Best way is setup the PPPOE Desktop and use the userID and password of BSNL to login and after that log into BSNL use your VPN connection to connection to your office network. Setup ICS in your computer where you use the PPPOE desktop to share the Internet with others computer on a LAN.