RestSharp JsonDeserializer is not correctly deserializing the response. Here is a comparison with Newtonsoft

RestSharp deserialization

using System.Linq;
using RestSharp;
using RestSharp.Serialization.Json;
using Xunit;

namespace JsonSerialization
{
    public class UnitTest1
    {
        private const string json = @"{""panes"":{""filter"":{""records"":[{""data"":{""customernumber"":""10002""}}]}}}";

        [Fact]
        public void newtonsoft()
        {
            var actual = Newtonsoft.Json.JsonConvert.DeserializeObject<FilterBaseModel>(json);

            Assert.Equal("10002", actual.Panes.Filter.Records.First().Data.Number);
        }

        [Fact]
        public void restsharp()
        {
            var actual = (new JsonDeserializer()).Deserialize<FilterBaseModel>(new RestResponse {Content = json});

            Assert.Equal("10002", actual.Panes.Filter.Records.First().Data.Number);
        }
    }
}

Where is your FilterBaseModel model?