How do I to use RealTime Firebase offline in Xamarin?

I am using the library FirebaseDatabase.net to use RealTime Firebase in Xamarin and it works very well when the device is online. Now I would like to save local data when cellphone is offline and when it turn back online the data should be synchronize automatically.

How could I to do this ?

So far I am trying this. It just working online

public class FirebaseService    {
        FirebaseClient firebase = new FirebaseClient("https://xxxxxxx-xxxx-xxxx.firebaseio.com/");
         
        public async Task AddContato(int id, string nome, string email){
            await firebase
               .Child("Contatos")
                  .PostAsync(new Contato() { Id = id, Nome = nome, Email = email });
        }

        public async Task<List<Contato>> GetContatos(){
            return (await firebase
              .Child("Contatos")
              .OnceAsync<Contato>()).Select(item => new Contato
              {
                  Nome = item.Object.Nome,
                  Email = item.Object.Email,
                  Id = item.Object.Id
              }).ToList();
        }


    }