Adding record number field to sorted results - doesn't work

The TV DATABASE can be downloaded freely FROM:
https://forums.thetvdb.com/viewtopic.php?f=3&t=7550

It IS a massive download at ~400MB AND expands TO ~2GB.

MariaDB [thetvdb]> SELECT @rownum:=@rownum+1 AS Num, res.* FROM (SELECT @rownum:=0) r, (SELECT id, SeriesName FROM tvseries WHERE SeriesName LIKE '%dallas%') res;
+------+--------+----------------------------------------------+
| Num  | id     | SeriesName                                   |
+------+--------+----------------------------------------------+
|    1 |  77092 | Dallas                                       |
|    2 |  79395 | Dallas Cowboys Cheerleaders: Making the Team |
|    3 |  79949 | Debbie Does Dallas Again                     |
|    4 |  80341 | Dallas SWAT                                  |
|    5 | 319215 | Little Women: Dallas                         |
|    6 | 203581 | Police Women OF Dallas                       |
|    7 | 242521 | Dallas (2012)                                |
|    8 | 250374 | Most Eligible Dallas                         |
|    9 | 252679 | The A-List: Dallas                           |
|   10 | 269228 | John Borrowmans Dallas                      |
|   11 | 271452 | Dallas Car Sharks                            |
|   12 | 274997 | Courtney Loves Dallas                        |
|   13 | 307475 | The REAL Housewives OF Dallas                |
|   14 | 292986 | Donna Decorates Dallas                       |
|   15 | 293986 | Dallas Divas & Daughters                     |
+------+--------+----------------------------------------------+
15 ROWS IN SET (0.10 sec)

MariaDB [thetvdb]> SELECT @rownum:=@rownum+1 AS Num, res.* FROM (SELECT @rownum:=0) r, (SELECT id, SeriesName FROM tvseries WHERE SeriesName LIKE '%dallas%' ORDER BY id,SeriesName ASC) res;
+------+--------+----------------------------------------------+
| Num  | id     | SeriesName                                   |
+------+--------+----------------------------------------------+
|    1 |  77092 | Dallas                                       |
|    2 |  79395 | Dallas Cowboys Cheerleaders: Making the Team |
|    3 |  79949 | Debbie Does Dallas Again                     |
|    4 |  80341 | Dallas SWAT                                  |
|    5 | 319215 | Little Women: Dallas                         |
|    6 | 203581 | Police Women OF Dallas                       |
|    7 | 242521 | Dallas (2012)                                |
|    8 | 250374 | Most Eligible Dallas                         |
|    9 | 252679 | The A-List: Dallas                           |
|   10 | 269228 | John Borrowmans Dallas                      |
|   11 | 271452 | Dallas Car Sharks                            |
|   12 | 274997 | Courtney Loves Dallas                        |
|   13 | 307475 | The REAL Housewives OF Dallas                |
|   14 | 292986 | Donna Decorates Dallas                       |
|   15 | 293986 | Dallas Divas & Daughters                     |
+------+--------+----------------------------------------------+
15 ROWS IN SET (0.04 sec)

MariaDB [thetvdb]> SELECT (@rn := @rn + 1) AS Num, t.id, t.SeriesName FROM tvseries t CROSS JOIN (SELECT @rn := 0) params WHERE t.SeriesName LIKE '%dallas%' ORDER BY t.id ASC;+------+--------+----------------------------------------------+
| Num  | id     | SeriesName                                   |
+------+--------+----------------------------------------------+
|    1 |  77092 | Dallas                                       |
|    2 |  79395 | Dallas Cowboys Cheerleaders: Making the Team |
|    3 |  79949 | Debbie Does Dallas Again                     |
|    4 |  80341 | Dallas SWAT                                  |
|    5 | 203581 | Police Women OF Dallas                       |
|    6 | 242521 | Dallas (2012)                                |
|    7 | 250374 | Most Eligible Dallas                         |
|    8 | 252679 | The A-List: Dallas                           |
|    9 | 269228 | John Borrowmans Dallas                      |
|   10 | 271452 | Dallas Car Sharks                            |
|   11 | 274997 | Courtney Loves Dallas                        |
|   12 | 292986 | Donna Decorates Dallas                       |
|   13 | 293986 | Dallas Divas & Daughters                     |
|   14 | 307475 | The REAL Housewives OF Dallas                |
|   15 | 319215 | Little Women: Dallas                         |
+------+--------+----------------------------------------------+
15 ROWS IN SET (0.05 sec)