SQL Server & SSRS working with parameters and multiple select values.

I think I need to use some variant of the split function or string_split or the c_split.
IN the SQL Stored Procedure I have declared the value
@location_code varchar (100)

I have referenced it in a table. l_location. The table is joined on. 
and I have it in the where clause ...AND ..tbl.location_code in (@location_code)

When the report runs in SSRS the lookup values show up and they can be selected individually or in multiples. Currently I can only select one value and it runs. If I select 2 or more the report shows no results but runs. That is why I think that I need to use split somewhere. I have tried many different ways and none of them worked. If anyone knows what I need to make sure I include please advise. Thank you. 

ALTER FUNCTION [dbo].[c_Split]
(
@InputString VARCHAR(MAX)
,@Separator VARCHAR(MAX)
)
RETURNS @ValueTable TABLE (Value VARCHAR(MAX))
AS
BEGIN

    DECLARE @SeparatorIndex INT, @TotalLength INT, @StartIndex INT, @Value VARCHAR(MAX)
    SET @TotalLength=LEN(@InputString)
    SET @StartIndex = 1

    IF @Separator IS NULL RETURN

    WHILE @StartIndex <= @TotalLength
    BEGIN
        SET @SeparatorIndex = CHARINDEX(@Separator, @InputString, @StartIndex)
        IF @SeparatorIndex > 0
        BEGIN
            SET @Value = SUBSTRING(@InputString, @StartIndex, @SeparatorIndex-@StartIndex)
            SET @StartIndex = @SeparatorIndex + 1
        END
        ELSE
        BEGIN
            Set @Value = SUBSTRING(@InputString, @StartIndex, @TotalLength-@StartIndex+1)
            SET @StartIndex = @TotalLength+1
        END
        INSERT INTO @ValueTable
        (Value)
        VALUES
        (@Value)
    END

    RETURN
END

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