Is Dragons Rescue Riders set in the same universe as the previous How to Train Your Dragon cartoons and films?

It's inspired by the same books (and the art style is directly related to films and TV series), but it's not canon with the main series of films and TV shows, according to Executive Producer Jack Thomas and Co-Executive Producer Brian K. Roberts.

Jack Thomas: The idea was to take the world and the DNA of the franchise and create a new type of storytelling aimed at a younger audience. That’s why it’s all new characters and all new locations, so that we could make it its own thing.

Jackson Murphy: And what elements did you take from the movies and the other TV series and incorporate into the new show?

Brian K. Roberts: We looked at a lot of the visuals from the movies and from “Race to the Edge” and other TV series and looked at ways that we could interpret that into a brighter, more chunky and a little younger-skewing visual… We created a world that felt younger, while still keying off of some of the same inspirations as the original films.

...

JT: I would say this: if you’re a fan of the franchise, and if you watch carefully, you will see Easter Eggs. But I’m not going to give you clues on where they are or what they are. (laughs) I love the main franchise. I did 78 episodes of “Race to the Edge” and before that “Defenders of Berk”. It’s really important to me that we fit in even though we are a very different aspect of the main franchise. So there are references.

_{^{INTERVIEW: “Dragons: Rescue Riders” EP Jack Thomas and Co-EP Brian K. Roberts}}