When the TWC/WU API was implemented in 2019, all queries have to have a valid API key (Free for WU data contributors, paid for other apps/non-contributors).
The author of underthewx would have to use an API key to RETRIEVE your WU conditions (either paid for by the author, or allow a setting for you to enter your PWS API key to use).
According to the apple store
UnderTheWX reads weather data off the popular AcuRite USB Professional Weather Centers and uploads it to Weather Underground, Weathercloud, PWSWeather, WOW and CWOP.
The app is a Status Bar Menu which displays Temperature, Humidity, Wind speed and direction, Rainfall and Barometer. UnderTheWX supports the AcuRite 01025, 01036, 02032, 02064 and 06037M USB Personal Weather Stations.
So is your problem with SUBMITTING data to WeatherUnderground? If so, you just need to have a WU userid/password as submissions are done with HTTP GET requests with the weather data incorporated in the URL along with your stationID, userid, password.