wunderground has been having lots of problems lately with a lot of things, web cam uploads being one of them
using a fixed IP just means you are using one particular server instead of the one allocated by load balancing
(and so then putting more strain on that server, etc)
if you have WD uploading the webcam image to a web site then you can set WU to get the image from that url instead
if you really want to though I can make it where you can set the IP