Message Store API - Sync Messages is bugged for ISync.

The "ISync" requires a syncToken. So, first you have to use the "FSync", get the response and parse the syncInfo to get the "syncToken" to be used with "ISync"

// first read
var syncMessagesParameters = new SyncMessagesParameters();
syncMessagesParameters.dateFrom = "2020-03-01T18:07:52.534Z";
syncMessagesParameters.dateTo = "2020-05-31T18:07:52.534Z";
var response = await rcsdk.Restapi().Account().Extension().MessageSync().Get(syncMessagesParameters);

Console.WriteLine(JsonConvert.SerializeObject(response));
// second read: parse the response to get the syncToken
var syncMessagesParameters = new SyncMessagesParameters();
syncMessagesParameters.syncType = new string[] { "ISync" };
syncMessagesParameters.syncToken = "paste the sync token here";
var response = await rcsdk.Restapi().Account().Extension().MessageSync().Get(syncMessagesParameters);

Console.WriteLine(JsonConvert.SerializeObject(response));

For your reference, please read this blog. It is not in C# but it gives you guidance of using the message store sync API.