There are quite a few instructional You-Tube videos for dryland swim training using stretch cords. I use them between swim sessions and they really make a difference! You can use them at home and no need to go to the gym! I stared out with a very low resistance band and performed repeated reps of 50 strokes or until I had relatively complete muscle fatigue . As my endurance increased, I went to a higher resistance stretch cord. After a winter layoff, I start again, beginning with the lightest resistance.
Stretch cords, are used by lots of swim coaches and collage swim teams. Inexpensive and proven results.