Stopped working a few days ago on my iPhone X as well (iOS 11.4). The app always indicates "There was a problem connecting". Interestingly, the The WU WEATHER widget still works on the iPhone X. I am certain there is a valid internet connection, no issues with any other app that depends on the internet, including the WU widget. My iPhone X is running Weather Underground app v5.11.6
On the same network, my five year old iPad running iOS 10.3.3 still works fine (it is to old to run iOS 11). The old iPad is running Weather Underground app v5.11.6 as well.
So it seems the issue is with iOS 11.4?
Did not realize how much I use this app every day until it stopped working, frustrating to say the least.