Not sure how to connect a separate rain gauge. I saw Nerto offers separate WiFi sensors too that measure ground humidity. My guess is that this is as good as a rainfall measurement.
Reading about your configuration maybe you can help me. I have just installed a Sprite too and I am trying to set Wunderground as provider in the app. I have an Alecto WS-5500 connected to Wunderground. When I select Wunderground as provider the app asks for my weather station ID which I have (ISTWIL10). But the app seems not to accept it as a valid ID. Do you have any suggestions?