How to extract dynamically data from another database in function with DBName as parameter?

I have one main database (MAIN) and several client DBs (CLIENTDB). I need to start query in MAIN and to get data from CLIENTDB. The client DB can be found also on linked server. My problem is that I need to do this in a function with EXEC, so I can get SELECT * FROM SRV.CLIENTDB, but this in not allowed by SQL Server. Is there another way to do that? I need to have code like inside the function: DECLARE @sSRV AS VARCHAR(128) = 'SRV'; DECLARE @sDB AS VARCHAR(128) = 'CLIENTDB1'; EXEC('SELECT * FROM ' + sSRV + '.' + @sDB + '.MyTable'); I need a function, because I need to make joins with it inside my engine.