in this video, it shows the use of tannin
It increases cross-linking, and makes the baits less soluble in water.
I also found that if you add fruit pectin to gelatin it raises the melting point temperature
An optimum tannin content around 10 wt% in gelatin–tannin films gives rise to strong, thermally stable, moisture resistant and flexible films as mechanical,
What is the bloom strength of the gelatin you are using?