Add missing month in a table of values
If you have a table of numbers, or can access the built-in table in master:
SELECT
    T1.id,
    Expanded.[date],
    Expanded.frequency
FROM 
(
    -- Add next date for the current id
    SELECT
        T.*,
        next_date = 
            LEAD(T.[date], 1) OVER (
                PARTITION BY T.id
                ORDER BY T.[date])
    FROM dbo.t AS T
) AS T1
CROSS APPLY
(
    -- All month ends &gt;= the current date and &lt; next date
    SELECT
        [date] = EOMONTH(T1.[date], SV.number),
        frequency = IIF(SV.number = 0, T1.frequency, 0),
        SV.number
    FROM master.dbo.spt_values AS SV
    WHERE
        SV.[type] = N'P'
        AND SV.number &lt; ISNULL(DATEDIFF(MONTH, T1.[date], T1.next_date), 1)
) AS Expanded
ORDER BY
    T1.id,
    T1.[date],
    Expanded.number;

The output matches that of the recursive CTE you provided (see demo link below):




id
date
frequency




1
2012-04-30
5


1
2012-05-31
0


1
2012-06-30
4


1
2012-07-31
0


1
2012-08-31
0


1
2012-09-30
0


1
2012-10-31
0


1
2012-11-30
0


1
2012-12-31
0


1
2013-01-31
0


1
2013-02-28
0


1
2013-03-31
0


1
2013-04-30
0


1
2013-05-31
0


1
2013-06-30
0


1
2013-07-31
25


2
2012-04-30
7


2
2012-05-31
4


2
2012-06-30
1


2
2012-07-31
6



Explanation
Logically, each row from T1 is expanded by the APPLY into the set of month-ends that ought to be present. Let's look at one particular row in the source table:




id
date
frequency




1
2012-04-30
5




Adding the LEAD in the table expression aliased to T1 gives:




id
date
frequency
next_date




1
2012-04-30
5
2012-06-30




This row is passed to the APPLY:
SELECT
    [date] = EOMONTH(T1.[date], SV.number),
    frequency = IIF(SV.number = 0, T1.frequency, 0),
    SV.number
FROM master.dbo.spt_values AS SV
WHERE
    SV.[type] = N'P'
    AND SV.number &lt; ISNULL(DATEDIFF(MONTH, T1.[date], T1.next_date), 1)

The references to columns in T1 are outer references. Substituting the values from the current T1 row for [date], next_date, and frequency gives:
SELECT
    [date] = EOMONTH('2012-04-30', SV.number),
    frequency = IIF(SV.number = 0, 5, 0),
    SV.number
FROM master.dbo.spt_values AS SV
WHERE
    SV.[type] = N'P'
    AND SV.number &lt; ISNULL(DATEDIFF(MONTH, '2012-04-30', '2012-06-30'), 1)

The ISNULL expression determines how many rows are returned from our table of numbers. In the current iteration the ISNULL expression evaluates to 2, so the rows returned from the numbers table have values 0 and 1.
The APPLY table expression as a whole returns:




date
frequency
number




2012-04-30
5
0


2012-05-31
0
1




Notice:

When number = 0, we are dealing with a row that exists in T1, so we just use the frequency given.
When number &gt; 0, the row does not exist in T1 so we need to return a zero for frequency.

That gives us the results needed for the single row we chose from T1. We can do the same for the next row from T1 (substituting values as before), and so on until the result is complete. For more on how APPLY works logically, see my article Understanding and Using APPLY.
The only special handling is for the last row per id, where the LEAD returns NULL. In that case, we only need one row from the numbers table (value zero) to process the current row from T1.

The built-in table is sufficient for a range of dates from '2000-01-01' to '2170-08-01'.
Try the db&lt;&gt;fiddle demo