mRemoteNG is not available for Mac but there are plenty of alternatives that runs on macOS with similar functionality. The best Mac alternative is RustDesk.
There is no centeralized free and opensource application on macos to enable connect to all windows and linux and also telnet service from single application.
It allows you to view all of your remote connections in a simple yet powerful interface and supports multiple protocols including RDP, VNC, SSH, HTTP/S and more ...