Smartthings ADT hub: how do I set up self-monitoring in the SmartThings app? (No paid plan)

I’ve installed a Smartthings ADT alarm hub (removed my old ST v2 hub first) and added the two ADT ST door sensors and a motion sensor which came with the ADT alarm hub, all of which works great using the ADT control panel. I started creating an dADT account until it go to the point of choosing a plan - I stopped there cos I don’t want ADT monitoring it as I want to do it myself. Any ideas welcome. The three sensors are all in the Smartthings app on my phone and work properly, but I can’t figure out how I can control the ADT alarm through the ST app. This is such a basic issue I feel daft even asking, but I’ve been trying to figure it out for two hours and I’m just stuck.