Does Rod of the Pact Keeper boost Spell Save DC of Ring of Shooting Stars?

The Rod of the Pact Keeper(+1) increases my Warlock’s spell save DC by 1. With an 18 Charisma at level 8 my spell save DC is 16. (8+3+4+1)

I just received a Ring of Shooting Stars. πŸΊπŸ‘ The ball lightning feature, and the shooting stars feature, require a DC 15 Dexterity save to avoid lightning or fire damage respectively. For both features, the same text is used.

That creature must make a DC 15 Dexterity saving throw.

Since my Warlock is attuned to the Rod of the Pact Keeper(+1), do they now have to make a DC 16 Dexterity saving throw to avoid the lightning or fire effects, or, is the DC 15 fixed and non-modifiable?

(A related question on the RoSS’s effects like faerie fire is not the same as this one).