Try adding tblassetcustom.SSHServer to the query.
Select Top 1000000 tblassets.AssetID,
tblassets.AssetName,
tsysassettypes.AssetTypename,
tsysassettypes.AssetTypeIcon10 As icon,
tblassets.IPAddress,
tblassetcustom.Model,
tblassets.OScode,
tblassets.Memory,
tblassets.ServiceVersion,
tblassets.Version,
tblassetcustom.SSHServer
From tblassets
Inner Join tblassetcustom On tblassets.AssetID = tblassetcustom.AssetID
Inner Join tsysassettypes On tsysassettypes.AssetType = tblassets.Assettype
Where tblassetcustom.Model = 'HP t520 Flexible Series TC' And
tblassetcustom.SSHServer <> N'' And tblassetcustom.State = 1