You can filter for DNS servers in your report by doing the following:
Join the tables containing service data:
INNER JOIN tblServices ON tblServices.AssetID = tblAssets.AssetID
INNER JOIN tblServicesUni ON tblServicesUni.ServiceuniqueID = tblServices.ServiceuniqueID
Add a check for the DNS Server service in your Where clause:
WHERE tblServicesUni.Caption LIKE '%DNS Server%'
AND tblServices.Started = 1