I had a similar issue with my iPad that started on IOS11 and continued on IOS12, the app worked fine on my iPhone. I spotted on another thread regarding another app that the solution for that app was to clear the application data by deleting the app via the storage menu (doesn't work if you just delete the app and re-install). (Settings, General iPad (iPhone) Storage). Wait a few minutes for the list to load, select AMPLIFI and Delete App (Offload app will not clear the cache and had no effect for me). Download the app again and login, this cured it for me.