On the Mac, open System Settings, then General, then Sharing, and turn on Screen Sharing. The same steps are in
the app under Settings, Set Up Your Mac.
Ruhmote can't find or connect to my Mac
Check that the iPhone or iPad and the Mac are on the same Wi-Fi network.
Allow Ruhmote to use the local network in iOS Settings, then Privacy & Security, then Local Network.
If a connection hangs, turn Screen Sharing off and on again on the Mac.
For a Mac on another network, use a private network such as Tailscale and add the Mac by its address.