"SIS" is "static ID server," which means that this controller (the hub, in your case) is the one responsible for handing out node IDs. What are you actually trying to accomplish? If you are trying to use your hub as your primary contoller and want to add a new Z-Wave device (like a switch, sensor, etc.) to your hub's network, then what you did in the end is correct: you do want to use the browser (or SmartStart in the app if you have devices that support it and happnen to prefer that route). This is the way that most people use their hub.
"Learn Mode" in the app is the wrong way to go for this. This is meant for adding the hub as a secondary controller to an existing network. It's technically something that is required by Z-Wave certification and is probably why Hubitat has this "feature," but I wouldn't recommend using the hub in this manner unless you really know what you're doing. And if you've already joined devices to the hub, you won't be able to use it this way at all (which I suspect is the root of your error).
