How to Get Router PIN?

I have the wifi password of my neighbour's router. Now I need to get the PIN of the router so that in case he changes the password, I'll be able to crack the newly set password through the PIN. I could have retrieved it through the default gateway IP but there's a dialogue box demanding username and password. I've tried all possible combinations but none of them works.

So, is there a way to get the PIN provided you know the WiFi password?