Here is the useful troubleshooting guide http://www.tp-link.com/en/article/?faqid=72.

This problem could be caused by the mismatching wireless channel on the wifi router and your wireless card. Change the wireless channel to be within in 1to 11 on the TL-WR740N.

Is the BSNL DSL router also a NAT device? If yes, you need to firstly open a port for your server on the TL-WR740N, and then open a port for the WAN IP on the DSL router. Here is the instruction on...

Give a try on different wireless channels on the router.

On your PC, are you able to ping 8.8.8.8? If you can ping 8.8.8.8 while you can not ping google.com, this could be a DNS problem.

Are you able to reach the server within the LAN network? Have you reserved a static IP address for your server? You may reopen the port for your server referring to...