Very noob, how to change WAN subnet mask

[stud]

I am sorry if this is very noob information but i cant seem to find this anywhere and my internet connection wont work without it,

How do i change the WAN subnet mask, it is set to 255.255.255.255 and needs to be 255.255.252.0

I have inputted everything else but this, and without my connection will not work, i am sorry if this is very simple but for the last two hours i cannot find out how to do it

Thanks

[stud]

never mind fixed  :)