The thing you are trying to do is definitely supported. I assume you are using config set admincode xxxxxxxxx before start <ip> <port>? If so, your issue is in the attempt to use a Thread Admin Passcode with the regular _meshcop._udp service. Thread Admin Passcodes can only be used with the ephemeral meshcop service that runs for a limited time after you obtain the Passcode. After you get the Passcode from the app look on your network for a _meshcop-e._udp service (notice the -e), which should be on the hub which you used the app to activate sharing mode on. Then simply note the port and do exactly what you have been doing but use the ephemeral service port when you execute start. Make sure to keep the UI with the Passcode on the app open until you have connected.
