How to detect the APP is block internet or not by firewall(2/3/4G network)

Hi, I face an issue that when my phone close the WiFi and turn on the 3/4G, when 3/4G network connected, i call function which will access an external API, my APP will crash!

i found there was a popup dialog "ABC application(my APP name) is blocked to access internet.", a button "Allow" show beside.
this dialog only can be see in debug model, and will be auto hidden in a few secends...

Crash come from this line:
AuthenticationResult authenticationResult = authenticationContext.AcquireTokenAsync(aadResource, credentials).Result;

My questions:
1) Is there any way to check the APP can access internet throught 2/3/4G(The phone alreay can access internet under 2/3/4G).
2) How can i call those dialog(use C#) which ask me is allow this APP can use 2/3/4G to access internet?

CrossConnectivity.Current.IsConnected; --> this plugin can only check the phone vs internet status, i need something check the APP vs internet status

i am new in Xamarin world, my APP just enabled Android, no IOS and Win phone, and current work fine in WiFi model.

Thanks in advance!