Little tip for anyone who decides to try this:
You need to enable IP access on the device you intend to connect to. It's under the security settings in RustDesk.
I've been playing around with it. The iOS RustDesk app is nice, and I've been controlling my Mac Mini at home using my iPad Pro with a Magic Keyboard, and it's shockingly smooth!
How secure is all this? Not really into networking and I always wondered if this has any risks.