Not Able to open github.com from my local network

Hello Everyone,

From one month whenever I am typing github.com in firefox browser then it shows
" Hmm. We’re having trouble finding that site.

We can’t connect to the server at github.com.

If that address is correct, here are three other things you can try:

Try again later.
Check your network connection.
If you are connected but behind a firewall, check that Firefox has permission to access the Web."

But when I try to open in other browser then it open but not authenticate with third party GUI tools (Git Desktop or sourcetree),

while I am able to do work very smoothly when my machine get IP dynamically from other router.

Please give suggestions what cloud be the root cause.