Nokia Lumia 520 does not remember WiFi?

Ok Starting from the beginning. I have 2 phones one is Nokia Lumia 520 & an Android Samsung Galaxy Prime. Now my neighbour gave his WiFi password to both my phones as he is a friend Note: He didnt told me the password just entered it so I can use. So now both my phones have wifi running smoothly. Now the problem starts my neighbour sometimes does something in his wifi router I dont know what and wifi is gone from my phones now Samsung Android shows SIGN IN TO WIFI and takes to a webpage saying to enter username and password and in Nokia Lumia 520 it is forgetting the wifi completely now after sometime my neighbour does some thing and wifi is back automatically in Android while Nokia Lumia 520 asks to re-enter the wifi password as if it was never entered before. I want to know how to solve this problem. Please help

Read more: Nokia Lumia 520 does not remember WiFi?