A Samsung phone also uses discovery over wifi to find it's available devices. To Roku, they can only cast certain apps. Chrome (and, recently, Edge) can Cast anything including tabs and desktops to a Chromecast on my TV. Chromecast discovery is through your wifi LAN, so it will see every device on your wifi or wired LAN.

Casting from Chrome to Roku (Chromecast) only works for certain apps (like YouTube) that have a Cast icon. My Roku Express is in the same room as the desktop so it is seen during discovery. Both are on the same wifi LAN, but they don't see each other as peer to peer for discovery. My computer is under a desk and probably has weak wifi anyway, while my Roku Ultra is in another room. However, the MiraCast discovery will ALWAY use the peer to peer method. Miracast does have the option to use your LAN if it gives it a better connection. Miracast creates a peer to peer network and does not normally use your WiFi or wired LAN. A Windows 10 Project to Roku connection uses MiraCast. Through some research and some experiments I found out some things that may helped me understand my problem better and may also help some of you.

Miracast windows 10 to roku tv