It's canonically established that Star Trek: Enterprise is set within the Prime Universe rather than the Kelvin Timeline, but there's an argument to be made that the show technically belongs in both.
The Kelvin Timeline debuted in the 2009 Star Trek reboot movie, directed by J.J. Abrams and starring Chris Pine as James T. Kirk. This movie replaced the original cast with new actors, but its biggest twist was that it didn't follow a traditional reboot format.
Instead, every other Star Trek TV show and movie that preceded it remained part of the main canon, only in another timeline. This was confirmed by the return of Leonard Nimoy as Spock Prime, alongside Zachary Quinto's new version of the character.
The return of Leonard Nimoy as Spock Prime alongside Zachary Quinto's new version of the legendary Vulcan confirmed this.
The show's placement in both timelines is a topic of discussion, with the Prime Universe being its originally intended setting.
Author's summary: Star Trek Enterprise in Kelvin Timeline explained.