The goal was to supply records to a jQuery DataTables (https://datatables.net/) by providing an API endpoint. Hence TParameters and DTResult, which mimics the datatables parameters). The data was stored in a separate database and I really didn't want to import it every week. Key issues this may help with: Routing, Getting a connection string from appsettings, adding "[FromForm]", and finally how to connect to the external database and query. The rest is pretty specific to jQuery DataTables Here's the Controller I came up with:

    [Route("/DT/DTApi/[action]")]
public class DTApiController : UmbracoApiController
{
    private readonly string _connectionString;
    public DTApiController(IConfiguration configuration)
    {
        _connectionString = configuration.GetConnectionString("ApplicationServicesBETA");
    }

    [HttpPost]
    public object StationsJunctions([FromForm]DTParameters param)
    {
        try
        {
            var dtsource = new List<StationJunction>();
            using (var db = new Database(_connectionString, DatabaseType.SqlServer2012, SqlClientFactory.Instance))
            {
                var query = new Sql().Select("State, OPName, Railroad, Name, R260Code, SPLC, FSAC, GeoArea, BEA, RateZipCode, SCAC, IntermodalRamp").From("v_Stations_Junctions");
                dtsource = db.Fetch<StationJunction>(query);
            }

            List<String> columnSearch = new List<string>();

            foreach (var col in param.Columns)
            {
                columnSearch.Add(col.Search.Value);
            }

            List<StationJunction> data = new StationJunctionResult().GetResult(param.Search.Value, param.SortOrder, param.Start, param.Length, dtsource, columnSearch);
            int count = new StationJunctionResult().Count(param.Search.Value, dtsource, columnSearch);
            DTResult<StationJunction> result = new DTResult<StationJunction>
            {
                draw = param.Draw,
                data = data,
                recordsFiltered = count,
                recordsTotal = count
            };
            return result;
        }
        catch (Exception ex)
        {
            return new { error = ex.Message };
        }
    }
}