CommunicationException while working with WCF service

Question

Microsoft Silverlight

Actually, you need to work with the database in SilverLight through the WCF service.
I connected to the database, created a LINQ to SQL model, and through the context of the 1st function I return a list of objects of the parent table: as a result, this error occurs.
The error lies in this function:

public ObservableCollection<Mod> EndGetMods(System.IAsyncResult result) {
    object[] _args = new object[0];
    ObservableCollection<Mod> _result = ((ObservableCollection<Mod>)(base.EndInvoke("GetMods", _args, result)));
    return _result;
            }

It is worth noting that Mod is the main table, there are 3 more child tables.
I tried to change the size of the transmitted message in the settings - it did not help.
I tried in all classes created by LINQ to SQL to write [DataContract] before the class name (and before one attribute created by LINQ to SQL) and [DataMember] before each of the Property - the result is still the same.
What other solutions are there? Or maybe it's better not to use LINQ to SQL? It is desirable to receive the data once by the big piece from this parent table.