I think that if your cellular data is disabled the result of this call will be Deregistered. Here's my example: Debug.WriteLine('Current network registration state is: ' + MobileBroadbandAccount.CreateFromNetworkAccountId(MobileBroadbandAccount.AvailableNetworkAccountIds).CurrentNetwork.NetworkRegistrationState) See the MobileBroadbandNetwork documentation here and the possible fields here.
Then call MobileBroadbandAccount.CreateFromNetworkAccountId on that network account id and try using MobileBroadbandNetwork.NetworkRegistrationState on that network. I'm not sure if this will do exactly what you want, but I think you can try calling MobileBroabandAccount.AvailableNetworkAccountIds and select the first in range to get your default network account id.
