Hey adrianhughes, thanks for the reply.
It was working before I sent my phone to nokia care. I took it back to them again after I found the wifi issue.
They formatted and installed latest clean firmware(41.0.*) but it didnt help. Wifi is still not working. They even did try reassembling the whole phone, but it didnt help either.
Nokia Care assistant told me that they'll need to send phone to company but it'll cost me too much so suggested me to rather buy a new phone as I've already paid them about $60 for repairing the strip between display & qwerty which was broken.
I guess I just have to rely on 3G as the phone is old & I'll just use it for few more months.