Hi! I use radicale!
I use it on android. For me, I have to use an app called "DAVx5." That app adds the calendar/contacts from my caldav server (radicale) to my phone. Then in my calendar/contacts apps I checkbox the appropriate calendar/contact list. It looks like you're on android... does this info help?
Radicale doesn't really have a GUI, all it has is a webpage that you can log in to. On that page you can download or upload your calendar/contacts in one file, but that's about it. The handling of everything is done by external apps.