Home
The "Add Server" dialog uses the same verification logic as the AOSP Settings app, which requires a valid domain name, most DoT resolvers should have one. For example the Cloudflare 1.1.1.1 resolver can be used by using one.one.one.one instead (or security.cloudflare-dns.com/family.cloudflare-dns.com for their malware/adult filtered resolvers) as described in the Set up 1.1.1.1 on Android documentation.
The "Off", "Auto", and "Private" modes directly correspond to the options provided by the system and available in the phone's settings app.
Android settings screenshot
While Off and Private are self explanatory, Auto is the built-in android Private DNS Automatic option (same one you can set using your phone's settings app), which is in turn called opportunistic in the RFC. As far as I understand the phone will attempt talking DoT to the server provided by the network config and will fallback to cleartext if that fails.