How can I replace the following query with ISNULL function in SQL?
This should do what you need

SELECT ISNULL(ta.[tag-value], 'N/A') AS [tag-value]

FROM   @AgentTableFilt ta


Or you could use the following

SELECT COALESCE(ta.[tag-value], 'N/A') AS [tag-value]

FROM   @AgentTableFilt ta