Link the computer through tblAssetRelations to tblAssets and filter the linked asset for monitors.
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetName,
tsysAssetTypes.AssetTypename,
tsysAssetTypes.AssetTypeIcon10 As icon,
tblAssets.IPAddress,
tblAssets.Lastseen,
tblAssets.Lasttried,
tblAssets1.AssetID AS MonitorAssetID,
tblAssets1.AssetName AS MonitorAssetName
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Inner Join tblAssetRelations On tblAssets.AssetID = tblAssetRelations.ParentAssetID
Inner Join tblAssets tblAssets1 On tblAssets1.AssetID = tblAssetRelations.ChildAssetID
Inner Join tsysAssetTypes tsysAssetTypes1 On tsysAssetTypes1.AssetType = tblAssets1.Assettype And tsysAssetTypes1.AssetTypename = 'Monitor'
Where tblAssetCustom.State = 1