unable to get AD groups from cross subdomain or child domain using AD C#

Hi ,
For one of my client their AD structure is like
Domain architecture example :

•      xyz.com (Main Domain)
1.      abc.xyz.com (Subdomain)
      abc.xyz/User1  --> user in abc sub domain
      abc.xyz/User2  -->user in abc sub domain
      ADGrp1  --> Ad group in this abc domain
      ADGrp2  --> Ad group in this abc domain


2.      test.xyz.com (Subdomain)
    test.xyz/User3    --> user in test sub domain
   test.xyz/User4    --> user in test sub domain
    ADGrp3  --> Ad group in this test domain
    ADGrp4  --> Ad group in this test domain

Now problem arises when in my web.config ADGrop3 is configured and and ADGrp3 consist od cross sub domain abc.xyz\user1 and user is not getting acces.

 private List<string> GetGroups()
        {
                  List<string> groups = new List<string>();
                  if (string.IsNullOrEmpty(domainName)) return groups;
           
            using (PrincipalContext principalContext = new PrincipalContext(ContextType.Domain, domainName))
            {
                try
                {
                    UserPrincipal user = UserPrincipal.FindByIdentity(principalContext, userName);
                    if (user != null)
                    {
                        PrincipalSearchResult<Principal> groupResults = user.GetGroups(principalContext);
                                    groups = getItems(groupResults);
                    }
                }
                catch (Exception ex)
                {
                    string errorMessage = "AD error: " + ex.Message;
                    if (ex.InnerException != null)
                    {
                        errorMessage += "with inner exception message: " + ex.InnerException.Message;
                    }

                    throw new xyz(errorMessage, ex);
                }
            }

            return groups;
        }


      private List<string> getItems(PrincipalSearchResult<Principal> itemResults , bool userItem =true)
            {
                  List<string> groupItems = new List<string>();
                  // expand to enumerator and enumerate manually rather than using LINQ or foreach
                  // this works around rare cases where enumerating through the groups throws an exception due to not handling an element in error correctly
                  // http://support.microsoft.com/kb/969166/en-us does not appear to correct this issue
                  IEnumerator<Principal> enumerator = itemResults.GetEnumerator();
                  bool success;

                  do
                  {
                        try
                        {
                              success = enumerator.MoveNext();
                        }
                        catch
                        {
                              break;
                              // truncate the returned results to whatever was retrieved before the failure. See
                              //https://blogs.msdn.com/b/dsadsi/archive/2010/12/14/title-system-directoryservices-accountmanagetment-userprincipal-getgroups-will-throws-comexception-error-0x5011-s-ads-errorsoccurred-while-enumerating-the-principalsearchresult-collection.aspx?Redirected=true
                              // - the failing element is likely the last one
                        }

                        if (success)
                        {
                              Principal userResult = enumerator.Current;
                              string addname = string.Empty;
                              if (userResult != null && !string.IsNullOrEmpty(userResult.Name))
                              {
                                    if (userItem)
                                    addname = userResult.Name.ToLower(CultureInfo.CurrentCulture);
                                    else
                                      addname = string.Format("{0}\\{1}", userResult.Context.Name.ToUpper(CultureInfo.CurrentCulture), userResult.SamAccountName.ToLower(CultureInfo.CurrentCulture));
                                    groupItems.Add(addname);
                              }
                        }
                  } while (success);
                  return groupItems;
            }