Use NOLOCK hint when calling table valued function
You cannot use a WITH (NOLOCK) on a Table-Valued Function, unless you put it on every single table in the code inside the function.

Your best bet would be, like you said, to SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED.

To change this back to the default, you need to find out what isolation level is currently set (before changing it above). 

This can be done by running DBCC USEROPTIONS and checking the value for isolation level.

The default tends to be READ COMMITTED, so to change it to that, you'll want to write a statement like the following:

SET TRANSACTION ISOLATION LEVEL READ COMMITTED