Parse series of strings into WHERE IN of stored procedure

Question

I'm trying to build an Excel tool that allows users to input a series of article IDs and query our SQL server for said IDs. Problem is that all of these IDs are strings, so I have to input them as '123', '456', '789' - which is obviously not working if I were to execute the SP via

exec SP_test @articleID = '123', '456', '789'

The SP itself is pretty basic:

ALTER PROCEDURE [dbo].[SP_test]
    @articleID as VARCHAR
AS
BEGIN

    SET NOCOUNT ON;

    SELECT
        ARTNR
        , ARTBEZ
    FROM table
    WHERE
        ARTNR IN 
            (@articleID)
END
GO

Like I said, this isn't working because it's not parsing '123' as '123' but rather 123.

I've then found a custom function to split strings in this thread: How to pass string parameter with `IN` operator in stored procedure SQL Server 2008

If I then just execute the query itself (not as a SP), it's actually giving me the proper results:

SELECT
    ARTNR
    , ARTBEZ
FROM table
WHERE
    ARTNR IN 
        (select value from fn_split('123, 456, 789', ','))

But if I then try to turn that into my SP a la

ALTER PROCEDURE [dbo].[SP_test]
    @articleID as VARCHAR
AS
BEGIN

    SET NOCOUNT ON;

    SELECT
        ARTNR
        , ARTBEZ
    FROM table
    WHERE
        ARTNR IN 
            (select value from fn_split(@articleID, ','))
END
GO

it seems to again be parsing the articleIDs incorrectly.

Can anyone clue me in on how to get this to work properly? I've seen the string_split function being thrown around in this context, but m server's compatibility level isn't high enough for that.