If you are using UnityUI , disable EventSystem at the start of your game so your UI is not interactable.
then check Application.internetReachability enum to see if internet is avilable and if so , then make you EventSystem enabled . Something like this:
void Awake(){
EventSystem.current.enabled = false;
TryConnectToInternet();
}
void TryConnectToInternet(){
if(Application.internetReachability == NetworkReachability.NotReachable){
//Display no intenet connection message and a try again
//button with click event : TryConnectToInternet
}
else
{
EventSystem.current.enabled = true;
}
}