Teracom modem wifi configuration in macbook pro

Hello,

I am using Macbook Pro and BSNL Teracom Modem, Now a days I connect to internet through Ethernet which works fine by the way, Thought of using the Wifi Option so as per the network providers advise tried to make a connection through wifi But I failed.
Then I referred many websites, Blogs but it din't give me anything good.
So Can you please help me to overcome this issue

Note: Four months back It(Wifi) was woking good.

Thanks in advance