It uses the 915MHz ISM radio band. Range is quite good compared to WiFi. I have a mositure sensor, also from Ecowitt and that is about 50 feet away and through three concrete foundation walls away from the gateway. It still shows full signal strength.
My rain gauge is only about 20 feet away from the gateway.
Do keep in mind Ecowitt makes their sensors and gateways in three different frequencies. Officially US only has the 915MHz equipment but you can order other frequencies from their website. But all the frequencies of the gateway and sensors must match with each other.