The relevent network_event_log ouput is:
[10:53:53.552] EVENT: network_connection_handler_impl.cc:614 Sending Connect Request to Shill: /cellular
[10:53:53.553] ERROR: network_connection_handler_impl.cc:676 Connect Failure, Shill error: org.freedesktop.DBus.Error.UnknownObject: /cellular
[10:53:53.554] ERROR: network_connection_handler.cc:98 Connect Failure: connect-failed: /cellular
[10:53:53.554] ERROR: network_connect.cc:206 Connect Failed: connect-failed: <UUID: 1>
[10:53:53.554] USER: network_connect.cc:470 ActivateCellular: <UUID: 1>


The code path that triggered this is:


NetworkConnectImpl::CallConnectToNetwork(network_id)
NetworkConnectionHandler::ConnectToNetwork(network_id)
...
NetworkConnectImpl::OnConnectFailed(network id)
NetworkConnectImpl::MaybeShowConfigureUIImpl(network_id)
NetworkConnectImpl::HandleUnconfiguredNetwork(network_id)
NetworkConnectImpl::ActivateCellular(network_id)
NetworkConnectImpl::ShowMobileSetup(network_id)
NetworkConnectDelegateChromeOS::ShowMobileSetupDialog(network_id)
MobileSetupDialog::ShowByNetworkId(network_id)

I do not see this log event from mobile_setup_ui:311:
NET_LOG_EVENT("Starting mobile setup", service_path);

This suggests that shill::kUsageURLProperty and shill::kPaymentPortalProperty are empty for the network.

We cache kPaymentPortalProperty as NetworkState.payment_url_, maybe we should test network->payment_url().empty() in NetworkConnectImpl::HandleUnconfiguredNetwork() and not attempt to show the dialog in that case. Is kUsageURLProperty useful by itself? We could cache that as well if we need to.

We should also test one or both of these properties in the WebUI and disable 'Connect' for Cellular of not activated and the necessary URL(s) are not available.

Ben, does that all make sense? I will be in MTV tomorrow and we can chat if necessary.