Enter the router's configuration, probably at address http://192.168.0.1/
If not that address, look in your documentation to get the correct address.
You probably need to enter the Advanced section and check for some websites filtering and remove Yahoo if it's listed.
Is your server using IPV4 or IPV6? As you are in India, and India (Asia in general) ran out of IPV4 addresses, the later is likely.
If you use IPV6, you may need to configure some IPV6 to IPV4 tunneling if Yahoo don't yet have an IPV6 IP address.
Electro
January 2012