Miracast is a mirroring protocol that uses the WiFi adapter, and more specifically WiFi Direct, to negotiate a connection. Even though you might be on the same network, it will always use the WiFi adapter for discovery first. 

As of version 2.6.0, it is no longer necessary to connect your Xbox One with an ethernet cable to your network in order for Miracast to work. However, it is still recommend for the best performance. Many Miracast discovery issues were also fixed in that version. Please make sure to update to the latest version.